[backport#16945#18181] refactor: introduce CChainState::GetCoinsCacheSizeState [plus fix]

Summary:
Core PR16945

02b9511d6bace5711e454d2b685b2fee0d65e341 tests: add tests for GetCoinsCacheSizeState (James O'Beirne)
b17e91d842724d2888a179a73585cc4c2ef1dc21 refactoring: introduce CChainState::GetCoinsCacheSizeState (James O'Beirne)

Pull request description:

This is part of the [assumeutxo project](https://github.com/bitcoin/bitcoin/projects/11):

Parent PR: #15606
Issue: #15605
Specification: https://github.com/jamesob/assumeutxo-docs/tree/master/proposal

This pulls out the routine for detection of how full the coins cache is from
FlushStateToDisk. We use this logic independently when deciding when to flush
the coins cache during UTXO snapshot activation ([see here](https://github.com/bitcoin/bitcoin/pull/15606/commits/231fb5f17e3bb8ce3323c559ef6f9b3aaa066543#diff-24efdb00bfbe56b140fb006b562cc70bR5275)).

Core PR18181

faca8eff39876cc8c0ee609a89bdcebc21976d47 test: Remove incorrect assumptions in validation_flush_tests (MarcoFalke)
fa31eebfe9107e14dc1d6b588f5b4c878d1010de test: Tabs to spaces in all tests (MarcoFalke)

Pull request description:

The tests assume standard library internals that may not hold on all supported archs or when the code is instrumented for sanitizer or debug use cases

Fixes #18111

Backport of Core PR16945 and PR18181 (the fix in PR18181 is contained to
test/validation_flush_tests.cpp)

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8938