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
```

1 files changed, 7 insertions, 2 deletions

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;