Improve coverage for CI

[shuds13]

Improving coverage as a part of #1268

Improves coverage in libE.py and gets close to full coverage for executor.py, mpi_runner.py and node_resources.py

Includes some code restructuring and minor fixes.

[codecov]

Codecov Report

Attention: Patch coverage is 97.43590% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.38%. Comparing base (786a83b) to head (0d99592).
Report is 55 commits behind head on develop.

Files	Patch %	Lines
libensemble/executors/mpi_runner.py	96.29%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1271       +/-   ##
============================================
+ Coverage    72.64%   94.38%   +21.74%     
============================================
  Files           90       90               
  Lines         8166     8262       +96     
  Branches      1459     1481       +22     
============================================
+ Hits          5932     7798     +1866     
+ Misses        1998      276     -1722     
+ Partials       236      188       -48     

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[jmlarson1]

I note that three re-tries in the CI (ubuntu-latest, openmpi, 2.5.2, 3.12, l) have failed in the same place in the test_executor.py unit test on this branch.

https://github.com/Libensemble/libensemble/actions/runs/8413803993/job/23054205905
https://github.com/Libensemble/libensemble/actions/runs/8413803993/job/23054933022
https://github.com/Libensemble/libensemble/actions/runs/8413803993/job/23036575193