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 eaeebfcb38)

tests/conftest.py

@@ -51,7 +51,7 @@ def atf_vars() -> Dict[str, str]:
     return ATFHandler.get_atf_vars()
 
 
-@pytest.mark.trylast
+@pytest.hookimpl(trylast=True)
 def pytest_configure(config):
     if config.option.help:
         return