strumpack: make standalone test for +mpi more robust

[tldahlgren]

Depends on #44874

@shahzebsiddiqui @wspear

This PR extends the work of #44874 to better support test_sparse_mpi processing from a node that cannot run srun (e.g., CI stacks, login nodes). It also does a little more tightening of the code and allows the sequential test to run even when +mpi.

It handles the srun issue with a timeout-like option (--immediate) that may or may not be supported but any problems are hopefully handled correctly by catching exceptions and allowing the search for an mpi alternative that is actually tied to the mpi used in the software's build.

I would appreciate feedback from others who have experienced problems running strumpack+mpi standalone tests:

• does this resolve your issues?
• does it create problems with Spack CI for the package?
• is the 30 second value for --immediate sufficient? appropriate?