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This PR adds the unit tests from #190 as it looks like that PR is likely to be withdrawn, at least for the time being, but the unit tests are still relevant to the main branch.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

All modified and coverable lines are covered by tests ✅

Project coverage is 76.94%. Comparing base (5efae61) to head (f6e2a84).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   73.29%   76.94%   +3.65%     
==========================================
  Files          25       25              
  Lines        1917     1917              
==========================================
+ Hits         1405     1475      +70     
+ Misses        512      442      -70     

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