Unit tests for step.save and step.skip

[emolter]

This PR adds the unit tests from #190 as it looks like that PR is likely to be withdrawn, at least for the time being, but the unit tests are still relevant to the main branch.

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • run regression tests with this branch installed ("git+https://github.com/<fork>/stpipe@<branch>")
    • jwst regression test
    • romancal regression test

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

Attention: Patch coverage is 95.76271% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.37%. Comparing base (1acb486) to head (1021d15).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_step.py	95.76%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   95.27%   95.37%   +0.09%     
==========================================
  Files          37       37              
  Lines        3177     3244      +67     
==========================================
+ Hits         3027     3094      +67     
  Misses        150      150              

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[braingram]

Thanks for putting this together. I left a few comments/suggestions.

[braingram]

Thanks!