Refactor MPI test to work with multi-slurmd and OpenMPI

OpenMPI can only run in multi-slurmd mode if no more than one node has
more than one task. Individual nodes with more than one task use shared
memory for communications and if more than one node is doing that, their
shared memory use collides. That means these MPI tests will work if five
nodes or more are available, otherwise some tests will fail. See test1.117
for a variation of this test that will work with OpenMPI and multi-slurmd
mode.