Let EkatCreateUnitTest use `mpiexec` (etc) for `nproc = 1` about ekat HOT 4 CLOSED

I agree. I'm not sure why we (I?) split the cmake flow for serial/parallel tests.

In SCREAM we have the cmake cache vars SCREAM_MPIRUN_EXE, SCREAM_MPIRUN_EXTRA_ARGS, and SCREAM_MPI_NP_FLAG, which can be used to tune the mpiexec invoke line (e.g., SCREAM_MPIRUN_EXE=srun for slurm).

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Sure, I'll add that stuff.

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That's a good point. I think we can live with setting the right MPIRUN command manually. So maybe this is just about removing the nproc>1 guards.

I can take care of this if we think that's true.

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EkatCreateUnitTest already has optional args for MPI_EXEC, MPI_EXTRA_ARGS, MPI_NP_FLAG. We do not have them in Ekat itself (so ekat internal testing does not allow to tweak them), but external apps can already achieve the result (except the nproc>1 guard, as you mentioned).

If you go ahead and do the nproc>1 guard mod, would you mind adding the cache vars EKAT_TEST_MPIRUN_EXE, EKAT_TEST_MPI_EXTRA_ARGS, and EKAT_TEST_MPIRUN_NP_FLAG in the tests/CMakeLists.txt file as well? They can of course default to the usual suspects: "mpirun", "", "-np" (though the extra args might be nice to have it defaulting to "--bind-to core", since I think that's usually what one needs).

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