Further enhance check_consistent.py

[AlexWaygood]

Add tests to make sure that:

• All files in test_cases are .py files.
• All .py files in test_cases have names starting with test_.
• All requirements listed in .pre-commit.config.yaml also appear in requirements-tests.txt
• All requirements listed in .pre-commit.config.yaml have the same version specifier as the ones in requirements-tests.txt

Also rename the check_same_files() function to check_no_symlinks(), which is more accurate these days.

[AlexWaygood]

This feels slightly fragile; not sure if there's a better way

[Akuli]

It seems to me that by convention, test_ means "you can run pytest or unittest or something over this directory". Should we instead enforce the opposite, no files named test_blah in our test cases directory that isn't really tests in the usual sense? Maybe not, because the non-test_ file names will show up in editors, and the test_ prefix avoids confusing them with stubs and scripts?

[AlexWaygood]

It seems to me that by convention, test_ means "you can run pytest or unittest or something over this directory". Should we instead enforce the opposite, no files named test_blah in our test cases directory that isn't really tests in the usual sense? Maybe not, because the non-test_ file names will show up in editors, and the test_ prefix avoids confusing them with stubs and scripts?

Hmm, interesting. I'm not wedded to having them be all be called test_foo.py. I mainly added the test here because it was easy to test for, and @sobolevn brought it up in #8520 and #8521.

I guess I think the most important thing is for us to be consistent in whether we call them test_foo.py or just foo.py.

[AlexWaygood]

If we wanted to establish a naming convention that distinguishes these files from the rest of typeshed, but also make clear that these aren't "tests" in the conventional sense, that you can run with pytest, maybe we could establish a naming convention with suffixes instead. Instead of test_foo.py, foo_tests.py?

[sobolevn]

Or typecheck_ 🤔

[AlexWaygood]

Or typecheck_ 🤔

Ooh I quite like that (though I'd go for the slightly less verbose check_, e.g. check_pow.py, etc.)