Silence pytest warning

[virtuald]

Without this change, I get the following warning when running pytest 5.3.5:

PytestUnknownMarkWarning: Unknown pytest.mark.mp_group_info - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
  PytestUnknownMarkWarning,