Register the marker for pytest-datafiles. This fixes the following
warning that we currently get:
.tox/py37-nocover/lib/python3.7/site-packages/_pytest/mark/structures.py:324
/Users/csingh43/buildstream/bst-tmp/.tox/py37-nocover/lib/python3.7/site-packages/_pytest/mark/structures.py:324: PytestUnknownMarkWarning: Unknown pytest.mark.datafiles - is this a typo? You can register custom marks to avoid this warning - for detail
s, see https://docs.pytest.org/en/latest/mark.html
PytestUnknownMarkWarning,