fix ctest MPI mpirun arguments

[tukss]

Some MPI implementations do not like -n 8 as a single argument to mpirun. Splitting it into two is more portable.

The CTest tests pass locally on my laptop but I haven't run any part of the test suite on a different machine.

Fixes #339.

Checklist

• Linting
  • Pre-commit run locally
• Building
  • CMake build does not produce any new warnings from changes in this PR
• Testing
  • Add a comment to the PR titled Testing with the following:
    • Which machines CTest unit tests
      have been run on and indicate that are all passing.
    • The Polaris omega_pr test suite
      has passed, using the Polaris e3sm_submodules/Omega baseline
    • Document machine(s), compiler(s), and the build path(s) used for -p for both the baseline (Polaris e3sm_submodules/Omega) and the PR build
    • Indicate "All tests passed" or document failing tests
    • Document testing used to verify the changes including any tests that are added/modified/impacted.
    • Performance related PRs: Please include a relevant PACE experiment link documenting performance before and after.

[mwarusz]

Like the linked issue says, most tests already use this pattern. Approve by inspection.

[grnydawn]

I also support this approach.