add unit testing for serial builds

[rem1776]

Description
Adds testing for mpi-less builds. It uses some scripting to conditionally skip all multi-core tests when built without MPI, so that it can just run a subset of existing tests to check functionality. This ends up being like 170 single-core tests, once any output checking tests are removed.

Includes some minor fixes to get the serial tests working. The majority of the file changes are just wrapping MPI calls or changing them to use the MPP equivalent for an MPI call, but there's also a couple of code changes for bugs that came up during serial tests.

Fixes #1829, but leaves out some requested changes to the mpirun function. Changing that will require changes to every test script, so deserves its own PR.

How Has This Been Tested?
Tested on the amd box with gcc 15 and intel 2025.2

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes