pytest: silence deprecation warning in the pytest atf wrapper

Convert pytest.mark decorator to the pytest.hookimpl() one,
as suggested by the
https://docs.pytest.org/en/latest/deprecations.html#configuring-hook-specs-impls-using-markers

Differential Revision: https://reviews.freebsd.org/D37884
MFC after: 2 weeks

(cherry picked from commit eaeebfcb38599c94e89bbba3e9c7aeb821c84d71)