don't put pytest marks on test fixtures

[drammock]

should prevent pytest warnings like this one:

======================================== warnings summary ========================================
mne/conftest.py:836
  /opt/mne/python/mne/conftest.py:836: PytestRemovedIn9Warning: Marks applied to fixtures have no effect
  See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function
    @pytest.fixture(scope="session")

../../mambaforge/envs/mnedev/lib/python3.11/site-packages/_pytest/mark/structures.py:357
  /opt/mambaforge/envs/mnedev/lib/python3.11/site-packages/_pytest/mark/structures.py:357: PytestRemovedIn9Warning: Marks applied to fixtures have no effect
  See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function
    store_mark(func, self.mark)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[drammock]

AFAICT this is the only test function that uses src_volume_labels, and since it's already marked with requires_testing_data I only copied over the slowtest mark.

[larsoner]

Thanks @drammock !