Merge bitcoin/bitcoin#33728: test: Add bitcoin-chainstate test for assumeutxo functionality

7b5d256af4 test: Add bitcoin-chainstate test for assumeutxo functionality (stringintech)
2bc3265649 Fix `ChainstateManager::AddChainstate()` assertion crash (stringintech)
5f3d6bdb66 Add regtest support to bitcoin-chainstate tool (stringintech)

Pull request description:

  This PR adds functional test coverage for the bitcoin-chainstate tool loading a datadir initialized with an assumeutxo snapshot.

  The PR also includes:
  - Fix for assertion crash in `ChainstateManager::AddChainstate()` when `prev_chainstate` has no initialized mempool (required for the test to pass)
  - `-regtest` flag support for bitcoin-chainstate to enable the testing

  This work started while experimenting with the bitcoin-chainstate tool and how the kernel API (#30595) behaved when loading a datadir containing assumeutxo data, during the time that PR was still under review. sedited suggested opening a PR to add this test coverage.

ACKs for top commit:
  achow101:
    ACK 7b5d256af4
  theStack:
    Concept and code-review ACK 7b5d256af4
  sedited:
    Re-ACK 7b5d256af4

Tree-SHA512: 5d3b0050cf2d53144b5f65451c991d5e212117b4541ae1368ecf58fde5f3cca4f018aad6ae32257b9ebb1c28b926424fbcff496ba5487cdc4eb456cea6db8b24

src/kernel/chainparams.cpp

@@ -638,7 +638,7 @@ public:
                 .blockhash = consteval_ctor(uint256{"385901ccbd69dff6bbd00065d01fb8a9e464dede7cfe0372443884f9b1dcf6b9"}),
             },
             {
-                // For use by test/functional/feature_assumeutxo.py
+                // For use by test/functional/feature_assumeutxo.py and test/functional/tool_bitcoin_chainstate.py
                 .height = 299,
                 .hash_serialized = AssumeutxoHash{uint256{"d2b051ff5e8eef46520350776f4100dd710a63447a8e01d917e92e79751a63e2"}},
                 .m_chain_tx_count = 334,