PyTest extensions to mark failing tests

[kmpaul]

Some tests fail on different platforms with different versions of MPI (MPICH, OpenMPI, etc.). We should provide a way of labeling these tests so they can be skipped. PyTest provides a @pytest.mark.skipif(...) decorator to tell PyTest to skip tests based on a given condition, and I think we want to figure out how to test for different versions and flavors of MPI.

For example, maybe something like this could be done:

import sys
from mpi4py import MPI

@pytest.mark.skipif(MPI.get_vendor()[0] == 'Open MPI' and
                    MPI.get_vendor()[1] < (4, 0, 1) and
                    sys.platform == 'darwin', reason="Doesn't work with OpenMPI 4.0.0 or lower on Macs")
def test_some_thing():
    ...

In this example would skip the test if the tests were being run on an Apple Mac (darwin) running with OpenMPI version 4.0.0 or lower.

...I think.

[kmpaul]

UPDATE: With PR #15, there no longer appear to be any differences in testing behavior between MPICH and OpenMPI. So, perhaps this feature is now a lower priority.