Do not trigger mpi if not needed in mpi.tester.
Do not trigger mpi if not needed in mpi.tester.

Only mpisub needs MPI. This is to allow running runtests.py via srun
at nersc.

nickhand/runtests,rainwoodman/runtests,rainwoodman/mpi4py_test,rainwoodman/runtests,nickhand/runtests,rainwoodman/mpi4py_test