Always run tests/test_mf6_disu_bmi.py::test_get_grid_nodes_per_face

[mwtoews]

I had intended to add a xfail marker for this test, but it seems to work as expected under Linux, so re-activating the test.

Also, add a RuntimeError when testing with other operating systems than Linux, Windows or macOS.

[codecov]

Codecov Report

Merging #77 (8512fde) into develop (21192db) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #77   +/-   ##
========================================
  Coverage    85.52%   85.52%           
========================================
  Files            6        6           
  Lines          442      442           
========================================
  Hits           378      378           
  Misses          64       64           

[Hofer-Julian]

Yes, I think we fixed that error, but forgot to update this specific test.

[Hofer-Julian]

Thanks!