Clean up files generated by tests

[WilliamJamieson]

This PR adds the use of the tmp_path fixture so that the files generated by tests get cleaned up at the end of the test.

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>/stcal@<branch>")
    • jwst regression test
    • romancal regression test

news fragment change types...

• changes/<PR#>.apichange.rst: change to public API
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.general.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.38%. Comparing base (d420770) to head (966a0cc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #316   +/-   ##
=======================================
  Coverage   86.38%   86.38%           
=======================================
  Files          49       49           
  Lines        8899     8899           
=======================================
  Hits         7687     7687           
  Misses       1212     1212           

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[braingram]

LGTM. Thanks!