testing: Replace _TestMethodWrapper with _callTestMethod

testing: Replace _TestMethodWrapper with _callTestMethod

Overriding _callTestMethod (which was introduced in python 3.8) is a
less hacky way to detect tests that fail to use ``@gen_test`` where
needed. It's not documented, but since Python 3.11 has introduced a
similar check to the standard library we'll be able to remove it in the
near future.

The major impetus for this change is an incompatibility with
Pytest 8.2, which has made a change that tries to instantiate test
classes at discovery time without an existing method name.

Fixes #3375
Closes #3374