test: Add tests with maleated snapshot data

Summary:
https://github.com/bitcoin/bitcoin/pull/19806/commits/769a1ef9fdc9c372f5bbe91d1961cabd60bc1895

test: Add tests with maleated snapshot data

https://github.com/bitcoin/bitcoin/pull/21681/files

validation: fix ActivateSnapshot to use hardcoded nChainTx

validation: remove nchaintx from assumeutxo metadata

Only test related changes for these commits are in this revision

https://github.com/bitcoin/bitcoin/pull/21582/commits/fa8fffebe8ac126f31143619843dd6578a2f4e3c

refactor: Prefer clean assert over UB in coinstats

https://github.com/bitcoin/bitcoin/pull/21582/commits/fa9b74f5ea89624e052934c48391b5076a87ffef

Fix assumeutxo crash due to missing base_blockhash

Only test related changes from this commit are in this revision.

https://github.com/bitcoin/bitcoin/pull/21584/commits/fae33f98e6a8d5934edbdce2eb8688112eac41a8

Fix assumeutxo crash due to invalid base_blockhash

Only test related changes from this commit are in this revision.

This is a backport of core#19806 [7/8] and test related changes from core#21681, core#21582 and core#21584

Depends on D11240

Test Plan:
ninja check

With UBSAN:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11241