test: Make blockencodings_tests deterministic

refactor: CBlockHeaderAndShortTxIDs constructor now always takes an explicit nonce.
test: Make blockencodings_tests deterministic using fixed seed providing deterministic
CBlockHeaderAndShortTxID nonces and dummy transaction IDs.

Fixes very rare flaky test failures, where the ShortIDs of test transactions collide, leading to
`READ_STATUS_FAILED` from PartiallyDownloadedBlock::InitData and/or `IsTxAvailable` giving `false`
when the transaction should actually be available.

 * Use a new `FastRandomContext` with a fixed seed in each test, to ensure 'random' uint256s
   used as fake prevouts are deterministic, so in-turn test txids and short IDs are deterministic
   and don't collide causing very rare but flaky test failures.
 * Add new test-only/internal initializer for `CBlockHeaderAndShortTxIDs` that takes a specified
   nonce to further ensure determinism and avoid rare but undesireable short ID collisions.
   In a test context this nonce is set to a fixed known-good value. Normally it is random, as
   previously.

Flaky test failures can be reproduced with:

```patch
diff --git a/src/blockencodings.cpp b/src/blockencodings.cpp
index 695e8d806a..64d635a97a 100644
--- a/src/blockencodings.cpp
+++ b/src/blockencodings.cpp
@@ -44,7 +44,8 @@ void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const {

 uint64_t CBlockHeaderAndShortTxIDs::GetShortID(const Wtxid& wtxid) const {
     static_assert(SHORTTXIDS_LENGTH == 6, "shorttxids calculation assumes 6-byte shorttxids");
-    return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
+    // return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0xffffffffffffL;
+    return SipHashUint256(shorttxidk0, shorttxidk1, wtxid) & 0x0f;
 }

```

to increase the likelihood of a short ID collision; and running

```shell
set -e;
n=0;
while (( n++ < 5000 )); do
    src/test/test_bitcoin --run_test=blockencodings_tests;
done
```

5 files changed, 38 insertions, 27 deletions

diff --git a/src/blockencodings.cpp b/src/blockencodings.cpp
index 5a4513d281..2bcb4b0c3d 100644
--- a/src/blockencodings.cpp
+++ b/src/blockencodings.cpp
@@ -17,8 +17,8 @@
 
 #include <unordered_map>
 
-CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block) :
-        nonce(GetRand<uint64_t>()),
+CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, const uint64_t nonce) :
+        nonce(nonce),
         shorttxids(block.vtx.size() - 1), prefilledtxn(1), header(block) {
     FillShortTxIDSelector();
     //TODO: Use our mempool prior to block acceptance to predictively fill more than just the coinbase
diff --git a/src/blockencodings.h b/src/blockencodings.h
index 01594db527..bc1d08ba5a 100644
--- a/src/blockencodings.h
+++ b/src/blockencodings.h
@@ -106,10 +106,15 @@ public:
 
     CBlockHeader header;
 
-    // Dummy for deserialization
+    /**
+     * Dummy for deserialization
+     */
     CBlockHeaderAndShortTxIDs() {}
 
-    CBlockHeaderAndShortTxIDs(const CBlock& block);
+    /**
+     * @param[in]  nonce  This should be randomly generated, and is used for the siphash secret key
+     */
+    CBlockHeaderAndShortTxIDs(const CBlock& block, const uint64_t nonce);
 
     uint64_t GetShortID(const Wtxid& wtxid) const;
 
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 6374cb52c1..a7b4452967 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -2151,7 +2151,7 @@ void PeerManagerImpl::BlockDisconnected(const std::shared_ptr<const CBlock> &blo
  */
 void PeerManagerImpl::NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& pblock)
 {
-    auto pcmpctblock = std::make_shared<const CBlockHeaderAndShortTxIDs>(*pblock);
+    auto pcmpctblock = std::make_shared<const CBlockHeaderAndShortTxIDs>(*pblock, GetRand<uint64_t>());
 
     LOCK(cs_main);
 
@@ -2549,7 +2549,7 @@ void PeerManagerImpl::ProcessGetBlockData(CNode& pfrom, Peer& peer, const CInv&
                 if (a_recent_compact_block && a_recent_compact_block->header.GetHash() == pindex->GetBlockHash()) {
                     MakeAndPushMessage(pfrom, NetMsgType::CMPCTBLOCK, *a_recent_compact_block);
                 } else {
-                    CBlockHeaderAndShortTxIDs cmpctblock{*pblock};
+                    CBlockHeaderAndShortTxIDs cmpctblock{*pblock, GetRand<uint64_t>()};
                     MakeAndPushMessage(pfrom, NetMsgType::CMPCTBLOCK, cmpctblock);
                 }
             } else {
@@ -6033,7 +6033,7 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
                         CBlock block;
                         const bool ret{m_chainman.m_blockman.ReadBlockFromDisk(block, *pBestIndex)};
                         assert(ret);
-                        CBlockHeaderAndShortTxIDs cmpctblock{block};
+                        CBlockHeaderAndShortTxIDs cmpctblock{block, GetRand<uint64_t>()};
                         MakeAndPushMessage(*pto, NetMsgType::CMPCTBLOCK, cmpctblock);
                     }
                     state.pindexBestHeaderSent = pBestIndex;
diff --git a/src/test/blockencodings_tests.cpp b/src/test/blockencodings_tests.cpp
index ff854c16eb..b0749c851c 100644
--- a/src/test/blockencodings_tests.cpp
+++ b/src/test/blockencodings_tests.cpp
@@ -27,23 +27,23 @@ static CMutableTransaction BuildTransactionTestCase() {
     return tx;
 }
 
-static CBlock BuildBlockTestCase() {
+static CBlock BuildBlockTestCase(FastRandomContext& ctx) {
     CBlock block;
     CMutableTransaction tx = BuildTransactionTestCase();
 
     block.vtx.resize(3);
     block.vtx[0] = MakeTransactionRef(tx);
     block.nVersion = 42;
-    block.hashPrevBlock = InsecureRand256();
+    block.hashPrevBlock = ctx.rand256();
     block.nBits = 0x207fffff;
 
-    tx.vin[0].prevout.hash = Txid::FromUint256(InsecureRand256());
+    tx.vin[0].prevout.hash = Txid::FromUint256(ctx.rand256());
     tx.vin[0].prevout.n = 0;
     block.vtx[1] = MakeTransactionRef(tx);
 
     tx.vin.resize(10);
     for (size_t i = 0; i < tx.vin.size(); i++) {
-        tx.vin[i].prevout.hash = Txid::FromUint256(InsecureRand256());
+        tx.vin[i].prevout.hash = Txid::FromUint256(ctx.rand256());
         tx.vin[i].prevout.n = 0;
     }
     block.vtx[2] = MakeTransactionRef(tx);
@@ -63,7 +63,8 @@ BOOST_AUTO_TEST_CASE(SimpleRoundTripTest)
 {
     CTxMemPool& pool = *Assert(m_node.mempool);
     TestMemPoolEntryHelper entry;
-    CBlock block(BuildBlockTestCase());
+    auto rand_ctx(FastRandomContext(uint256{42}));
+    CBlock block(BuildBlockTestCase(rand_ctx));
 
     LOCK2(cs_main, pool.cs);
     pool.addUnchecked(entry.FromTx(block.vtx[2]));
@@ -71,7 +72,7 @@ BOOST_AUTO_TEST_CASE(SimpleRoundTripTest)
 
     // Do a simple ShortTxIDs RT
     {
-        CBlockHeaderAndShortTxIDs shortIDs{block};
+        CBlockHeaderAndShortTxIDs shortIDs{block, rand_ctx.rand64()};
 
         DataStream stream{};
         stream << shortIDs;
@@ -128,8 +129,8 @@ public:
         stream << orig;
         stream >> *this;
     }
-    explicit TestHeaderAndShortIDs(const CBlock& block) :
-        TestHeaderAndShortIDs(CBlockHeaderAndShortTxIDs{block}) {}
+    explicit TestHeaderAndShortIDs(const CBlock& block, FastRandomContext& ctx) :
+        TestHeaderAndShortIDs(CBlockHeaderAndShortTxIDs{block, ctx.rand64()}) {}
 
     uint64_t GetShortID(const Wtxid& txhash) const {
         DataStream stream{};
@@ -146,7 +147,8 @@ BOOST_AUTO_TEST_CASE(NonCoinbasePreforwardRTTest)
 {
     CTxMemPool& pool = *Assert(m_node.mempool);
     TestMemPoolEntryHelper entry;
-    CBlock block(BuildBlockTestCase());
+    auto rand_ctx(FastRandomContext(uint256{42}));
+    CBlock block(BuildBlockTestCase(rand_ctx));
 
     LOCK2(cs_main, pool.cs);
     pool.addUnchecked(entry.FromTx(block.vtx[2]));
@@ -156,7 +158,7 @@ BOOST_AUTO_TEST_CASE(NonCoinbasePreforwardRTTest)
 
     // Test with pre-forwarding tx 1, but not coinbase
     {
-        TestHeaderAndShortIDs shortIDs(block);
+        TestHeaderAndShortIDs shortIDs(block, rand_ctx);
         shortIDs.prefilledtxn.resize(1);
         shortIDs.prefilledtxn[0] = {1, block.vtx[1]};
         shortIDs.shorttxids.resize(2);
@@ -216,7 +218,8 @@ BOOST_AUTO_TEST_CASE(SufficientPreforwardRTTest)
 {
     CTxMemPool& pool = *Assert(m_node.mempool);
     TestMemPoolEntryHelper entry;
-    CBlock block(BuildBlockTestCase());
+    auto rand_ctx(FastRandomContext(uint256{42}));
+    CBlock block(BuildBlockTestCase(rand_ctx));
 
     LOCK2(cs_main, pool.cs);
     pool.addUnchecked(entry.FromTx(block.vtx[1]));
@@ -226,7 +229,7 @@ BOOST_AUTO_TEST_CASE(SufficientPreforwardRTTest)
 
     // Test with pre-forwarding coinbase + tx 2 with tx 1 in mempool
     {
-        TestHeaderAndShortIDs shortIDs(block);
+        TestHeaderAndShortIDs shortIDs(block, rand_ctx);
         shortIDs.prefilledtxn.resize(2);
         shortIDs.prefilledtxn[0] = {0, block.vtx[0]};
         shortIDs.prefilledtxn[1] = {1, block.vtx[2]}; // id == 1 as it is 1 after index 1
@@ -269,10 +272,11 @@ BOOST_AUTO_TEST_CASE(EmptyBlockRoundTripTest)
     CMutableTransaction coinbase = BuildTransactionTestCase();
 
     CBlock block;
+    auto rand_ctx(FastRandomContext(uint256{42}));
     block.vtx.resize(1);
     block.vtx[0] = MakeTransactionRef(std::move(coinbase));
     block.nVersion = 42;
-    block.hashPrevBlock = InsecureRand256();
+    block.hashPrevBlock = rand_ctx.rand256();
     block.nBits = 0x207fffff;
 
     bool mutated;
@@ -282,7 +286,7 @@ BOOST_AUTO_TEST_CASE(EmptyBlockRoundTripTest)
 
     // Test simple header round-trip with only coinbase
     {
-        CBlockHeaderAndShortTxIDs shortIDs{block};
+        CBlockHeaderAndShortTxIDs shortIDs{block, rand_ctx.rand64()};
 
         DataStream stream{};
         stream << shortIDs;
@@ -306,15 +310,17 @@ BOOST_AUTO_TEST_CASE(EmptyBlockRoundTripTest)
 BOOST_AUTO_TEST_CASE(ReceiveWithExtraTransactions) {
     CTxMemPool& pool = *Assert(m_node.mempool);
     TestMemPoolEntryHelper entry;
-    const CBlock block(BuildBlockTestCase());
-    std::vector<CTransactionRef> extra_txn;
-    extra_txn.resize(10);
+    auto rand_ctx(FastRandomContext(uint256{42}));
 
     CMutableTransaction mtx = BuildTransactionTestCase();
-    mtx.vin[0].prevout.hash = Txid::FromUint256(InsecureRand256());
+    mtx.vin[0].prevout.hash = Txid::FromUint256(rand_ctx.rand256());
     mtx.vin[0].prevout.n = 0;
     const CTransactionRef non_block_tx = MakeTransactionRef(std::move(mtx));
 
+    CBlock block(BuildBlockTestCase(rand_ctx));
+    std::vector<CTransactionRef> extra_txn;
+    extra_txn.resize(10);
+
     LOCK2(cs_main, pool.cs);
     pool.addUnchecked(entry.FromTx(block.vtx[2]));
     BOOST_CHECK_EQUAL(pool.get(block.vtx[2]->GetHash()).use_count(), SHARED_TX_OFFSET + 0);
@@ -326,7 +332,7 @@ BOOST_AUTO_TEST_CASE(ReceiveWithExtraTransactions) {
     BOOST_CHECK_EQUAL(pool.get(block.vtx[1]->GetHash()), nullptr);
 
     {
-        const CBlockHeaderAndShortTxIDs cmpctblock{block};
+        const CBlockHeaderAndShortTxIDs cmpctblock{block, rand_ctx.rand64()};
         PartiallyDownloadedBlock partial_block(&pool);
         PartiallyDownloadedBlock partial_block_with_extra(&pool);
 
diff --git a/src/test/fuzz/partially_downloaded_block.cpp b/src/test/fuzz/partially_downloaded_block.cpp
index 2bf47930f4..0c874c76e8 100644
--- a/src/test/fuzz/partially_downloaded_block.cpp
+++ b/src/test/fuzz/partially_downloaded_block.cpp
@@ -50,7 +50,7 @@ FUZZ_TARGET(partially_downloaded_block, .init = initialize_pdb)
         return;
     }
 
-    CBlockHeaderAndShortTxIDs cmpctblock{*block};
+    CBlockHeaderAndShortTxIDs cmpctblock{*block, fuzzed_data_provider.ConsumeIntegral<uint64_t>()};
 
     CTxMemPool pool{MemPoolOptionsForTest(g_setup->m_node)};
     PartiallyDownloadedBlock pdb{&pool};