allowing runs of several mpi routine tests

Spawning MPI processes consumes slots, and I have not figured out how to
properly free them. Instead, I create a hostfile for openmpi that has 24
slots. While this incurs severe performance losses in production (see
openmpi FAQ), it is fine for tests. One alternative would be to spawn
the processes in "oversubscribe" mode, but I could not find a way to do
that.