[CMake] oversubscribing OpenMPI in tests

[Zaak Beekman]

Does anyone know a good way to determine if `find_package(MPI REQUIRED)` is
returning OpenMPI? I need to know, because OpenMPI does not handle
oversubscribed (more ranks than cores) tests well without explicitly
telling OpenMPI that you are oversubscribing. This can be done by creating
a hosts file, and passing some flags to `mpirun` to tell it to use the
hosts file, or can be accomplished by passing the `--oversubscribe` flag.
The problem is that MPICH has different flags/host-file syntax.

(I am oversubscribing some of the project's tests because this will often
find correctness problems due to race conditions or other parallel
programming errors that are not always exposed otherwise.)
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