gh-136438: Make sure `test_generated_cases` pass with all optimization levels (#136594)

Fix the `test_generated_cases` to work with `-O` or `-OO` flags.
Previously, `test_generated_cases` was catching an `AssertionError` while `Tools/cases_generator/optimizer_generator.py` used an `assert` statement.  This approach semantically incorrect, no one should trying to catch an `AssertionError`!
Now the `assert` statement has been replaced with an explicit `raise ValueError(...)` and the corresponding `self.assertRaisesRegex(AssertionError, ...)` has been updated to catch a `ValueError` instead.