test: subTest creates more problems than it solves with pytest >=9

[cpcloud]

As of pytest 9 with pytest-xdist, the subTest method of unittest.TestCase now runs subtests across processes. This creates a requirement that subtest inputs must be process-serializable, which isn't satisified for various numba types.

A solution to this problem is to move towards pytest.mark.parametrize, which has no such requirement. I looked at our subTest usage, and AFAICT all of it parametrizes over static values, meaning it's a straightforward conversion.

The annoying thing is pytest will not parametrize unittest.TestCase test methods subclasses, only methods of classes that are not subclasses of TestCase.

I think this means that we need to move away from TestCase subclasses entirely, and unittest as well.

I think we can do this by moving the CUDATestCase setup and teardown to session/module/function fixtures as needed, and move the methods that it has to top level functions that can be imported from conftest.py.

[cpcloud]

For now, I'm going to set an upper bound on pytest to <9 so that we can continue to update other dependencies.

[gmarkall]

See also #619. I think there is an extension that allows parametrization of test classes: https://github.com/adamchainz/unittest-parametrize but I don't yet have a perspective on whether it's worth introducing instead of just converting the test suite to pytest.

[cpcloud]

I think we can preserve most of the test suite structure (e.g., not rewriting everything to function tests instead of classes) without that plugin.

That said, I don't think it ultimately matters that much since if we have to move away from subtest, then adding a new plugin that has the equivalent behavior to pytest.mark.parametrize seems like approximately the same amount of work as using the native decorator.

[cpcloud]

In which case, not adding a dependency seems preferable.