Add functionality for testing separate PME nodes

GMX_TEST_NPME plus either kind of MPI triggers a test with three MPI
ranks including one PME-only rank. Manages gpuid correctly for all
test cases.

Change-Id: Ia6853f096b40d5a095a263032367eb6c9a746831
Merge "Add functionality for testing separate PME nodes" into 5.0.0