FIX: Removes warnings about unknown pytest marker (#715)
This is a low prio PR but it solves an annoyance.
Right now, when running tests, the output is spammed by messages like:
> PytestUnknownMarkWarning: Unknown pytest.mark.multi_gpu_tests - is
this a typo? ...
This makes it more difficult to see the actually relevant information.
This PR fixes this by registering the two pytest markers we use, thus
removing the warnings.
