Improve README in test_cases directory

[AlexWaygood]

This is basically a TODO for me (though if somebody else wants to help out, they're welcome!).

• Different test cases serve different purposes -- the tests in test_logging and test_object are pretty different to the tests in test_pow. Explain the different purposes of these tests in the README.
• Document that PEP 585 can't be used in the test_cases directory (unless from __future__ import annotations is at the top of the file). At the moment it says you can't always use new syntax in stubs, but the only examples given are PEP 604-related.
• Document use of # type: ignore'd lines (in combination with pyright: reportUnnecessaryTypeIgnoreComment=true and mypy's --warn-unused-ignore settings) to check for cases where we want a type checker to report an error. This can maybe just be a link to https://typing.readthedocs.io/en/latest/source/quality.html#testing-using-assert-type-and-warn-unused-ignores