Only run test_derecho_mpirun Python test on Derecho

[samsrabin]

However, it should throw a warning on non-Derecho systems telling the user to make sure and test on Derecho too.

[samsrabin]

Or maybe this should be moved out of our Python testing entirely—maybe even out of our repo? @ekluzek would it make sense for this to be a CIME test instead?