Register pytest.mark.btrfs
Avoids warnings.
tests/test_reflink.py:60
tests/test_reflink.py:60: PytestUnknownMarkWarning: Unknown pytest.mark.btrfs - is this a typo? You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/mark.html
@pytest.mark.btrfs