Some C-API tests use `assert`, so they don't run in non-debug mode

[encukou]

The assert function in C does nothing in non-debug builds.

Like assert vs. self.assertTrue in Python, tests (in Modules/_testcapimodule.c, Modules/_testapi/* and similar) should not use assert. They should fail regardless of build settings.

Possible fixes for this kind of issue:

• Raise PyExc_AssertionError or similar explicitly
• Move the test to Python if possible (or just part of it, but splitting a test up between files has a readability cost)
• Even Py_FatalError would be better than assert

[encukou]

Ah, _testcapimodule.c has #undef NDEBUG to enable assert. The parts that are being split out just need it too. I'll tie the fix to #93649 rather than this issue.