Nexus: no longer require mpirun for test passage

[jtkrogel]

Proposed changes

Addresses #3120. Nexus tests no longer strictly require mpirun. If mpirun is detected in the path, it is executed in the tests ("mpirun -np 1 echo run"). Otherwise a serial test is performed ("echo run").

What type(s) of changes does this code introduce?

• Bugfix

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Laptop

Checklist

• Yes. This PR is up to date with current the current state of 'develop'

[qmc-robot]

Can one of the admins verify this patch?

[prckent]

Thanks Jaron. I think this should be a more than good enough fix for the problems we have been seeing. It is simpler and more elegant than what I was envisaging (command line option) so I think we should go with your solution.

[ye-luo]

Test this please

[williamfgc]

Fixed unstable spock nightly test, nice!