test: Log to debug.log in all unit tests #16252

This makes it easier to debug a frozen test or a test that failed. To debug a failed test, remove the line fs::remove_all(m_path_root);.

The pull is done in three commits:

• Create a datadir for every unit test once (and only once). This requires the SetDataDir function to go away.
• Use the common setup in the gui unit tests. Some of those tests are testing the init sequence, so we'd have to undo some of what the testing setup did.
• Log to the debug.log in all tests

Concept ACK. I think logging can be useful for tests.

To be clear: this creates a temporary data directory for debug.log right, it doesn't touch the default data directory?

Yes, that should absolutely never happen, with or without this pull request. See also:

https://github.com/bitcoin/bitcoin/blob/413e438ea9767710d4810c4400fd1242ca52cd1c/.travis/test_05_before_script.sh#L9

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #16278 (tests: Reduce compilation time and unneccessary recompiles by removing unused includes in tests by practicalswift)
• #16273 (refactor: Reduce total compilation time by 2% and avoid unnecessary recompiles by removing unused includes by practicalswift)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

code review, tested that debug.logs are being created under ./test_common_Bitcoin Core/*/debug.log ACK fabc57e07dc34c103552cb51c9277bb48ef97739