use @classmethod per pytest guidance

use @classmethod per pytest guidance

Fixed class-scoped pytest fixtures that were defined as instance methods
using ``self``, which is deprecated as of pytest 9.1 and will be removed in
pytest 10. Fixtures are now decorated with a compatibility ``@classmethod``
decorator and use ``cls`` as the first parameter.

Fixes: #13392
Change-Id: I866de54e22569c9d55eb97dce165670994ec4a57