gh-93649: Split unicode tests from _testcapimodule.c & add some more

[encukou]

• Move PyUnicode tests to a separate file
• Add some more tests for PyUnicode_FromFormat

This is the refactoring & tests for existing functionality from #95505, leaving the change (and its tests) for another PR.

In addition to the changes in #95505, I made CHECK_FORMAT_* to check for SystemError when it gets NULL as the expected result. Some tests need that, as the behavior changed in #95784.

• Issue: Split up _testcapimodule.c #93649

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 995e666 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[miss-islington]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[miss-islington]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.10.
🐍🍒⛏🤖

[miss-islington]

Sorry, @encukou, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 325ae93b6b7113cd4a4c2ce441615ae7def779e2 3.11

[miss-islington]

Sorry @encukou, I had trouble checking out the 3.10 backport branch.
Please retry by removing and re-adding the "needs backport to 3.10" label.
Alternatively, you can backport using cherry_picker on the command line.
cherry_picker 325ae93b6b7113cd4a4c2ce441615ae7def779e2 3.10

[serhiy-storchaka]

@encukou, could you please backport this PR to 3.11 and 3.10? It is necessary for backporting #99613, #99651, #99868, and others.

[encukou]

I'd rather not -- _testcapi is only split up in 3.12. It would need backporting at least some of the other PRs in #93649.
I can rebase #99613 instead, would that help?

[serhiy-storchaka]

Are you volunteering to backport also #99651, #99868 and future tests?

[encukou]

I can put them on my TODO list.
The C changes are mostly about adding new functions, aren't they?

[serhiy-storchaka]

I think that it would be less work to split _testcapi in 3.11 and 3.10, so I will try this way.