Merge #20323: tests: Create or use existing properly initialized NodeContexts

81137c60fe6234569e1c5e6760f3a6f016956944 test: Add new ChainTestingSetup and use it (Carl Dong)
7e9e7fe56734d729ed7de39e880577b135dfd368 qt/test: [FIX] Add forgotten Context setting in RPCNestedTests (Carl Dong)

Pull request description:

  This is part 1/n of the effort to [de-globalize `ChainstateManager`](https://github.com/bitcoin/bitcoin/pull/20158)

  Reviewers: Looking for tested/Code-Review/plain-ACKs

  ### Context

  In many of our tests, we manually instantiate `NodeContext`s or `ChainstateManager`s in the test code, which is error prone. Instead, we should create or use existing references because:
  1. Before we [de-globalize `ChainstateManager`](https://github.com/bitcoin/bitcoin/pull/20158), much of our code still acts on `g_chainman` (our global `ChainstateManager`), sometimes even when you're calling a method on a specific instance of `ChainstateManager`! This means that we may act on two instances of `ChainstateManager`, which is most likely not what we want.
  2. Using existing references (initialized by the `{Basic,}TestingSetup` constructors) means that you're acting on objects which are properly initialized, instead of "just initialized enough for this dang test to pass". Also, they're already there! It's free!
  3. By acting on the right object, we also allow the review-only assertions in future commits of [de-globalize `ChainstateManager`](https://github.com/bitcoin/bitcoin/pull/20158) to work and demonstrate correctness.

  Some more detailed debugging notes can be found in the first commit, reproduced below:
  ```
  Previously, the validation_chainstatemanager_tests test suite
  instantiated its own duplicate ChainstateManager on which tests were
  performed.

  This wasn't a problem for the specific actions performed in
  that suite. However, the existence of this duplicate ChainstateManager
  and the fact that many of our validation static functions reach for
  g_chainman, ::Chain(state|)Active means we may end up acting on two
  different CChainStates should we write more extensive tests in the
  future.

  This change adds a new ChainTestingSetup which performs all
  initialization previously done by TestingSetup except:

  1. Mempool sanity check frequency setting
  2. ChainState initialization
  3. Genesis Activation
  4. {Ban,Conn,Peer}Man initialization

  Means that we will no longer need to initialize a duplicate
  ChainstateManger in order to test the initialization codepaths of
  CChainState and ChainstateManager.

  Lastly, this change has the additional benefit of allowing for
  review-only assertions meant to show correctness to work in future work
  de-globalizing g_chainman.

  In the test chainstatemanager_rebalance_caches, an additional
  LoadGenesisBlock call is added as MaybeReblanaceCaches eventually calls
  FlushBlockFile, which tries to access vinfoBlockFile[nLastBlockFile],
  which is out of bounds when LoadGenesisBlock hasn't been called yet.

  -----

  Note for the future:

  In a previous version of this change, I put ChainTestingSetup between
  BasicTestingSetup and TestingSetup such that TestingSetup inherited from
  ChainTestingSetup.

  This was suboptimal, and showed how the class con/destructor inheritance
  structure we have for these TestingSetup classes is probably not the
  most suitable abstraction. In particular, for both TestingSetup and
  ChainTestingSetup, we need to stop the scheduler first before anything
  else. Otherwise classes depending on the scheduler may be referenced
  by the scheduler after said classes are freed. This means that there's
  no clear parallel between our teardown code and C++'s destructuring
  order for class hierarchies.

  Future work should strive to coalesce (as much as possible) test and
  non-test init codepaths and perhaps structure it in a more fail-proof
  way.
  ```

ACKs for top commit:
  MarcoFalke:
    ACK 81137c60fe looking excellent now 🐩
  jnewbery:
    ACK 81137c60fe6234569e1c5e6760f3a6f016956944
  ryanofsky:
    Code review ACK 81137c60fe6234569e1c5e6760f3a6f016956944. This change is simpler after the rebase because wallet & bench commits are dropped.

Tree-SHA512: a8d84f08f2db6428b0b88449bdc814c9db35b7559156d536dfebd3225c2707dba65959e76d2152e3f8c96eacbf1e0b0000f745edf1e196deddb97ff1ef360953

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