Improve and expand executor focused testing.
Commit is focused primarily on testing the executor (both shell and batch) found in the Jacamar application.
- Improve Makefile and scripting around Docker/Pavilion testing.
- Add structure for testing Slurm via container. Further expansion of test steps required.
- Limit testing in CI to manual trigger and when MR target main to avoid wasting limited free CI minutes.
- Unit tests for batch executor Prepare() and Cleanup() functionality.
Minor code enhancements align all supported batch executor (Cobalt, Slurm, and LSF) in how they handle the prepare_exec stage.