Update exp_to_source unit tests to use mock data instead of loading from file

[emolter]

Required before spacetelescope/stdatamodels#539 can be merged

This PR updates the unit tests of exp_to_source to take in mock data instead of loading fits files from tests/data/. Mocking the data is a better pattern for unit tests.

This is required to move forward with the removal of the unused V23ToSky transform model, which was replaced by Astropy standard models about 5 years ago (#5206), because the test files are old enough that their WCS still relies on V23ToSky.

There is still open discussion about whether that removal should happen, but these test updates are a good idea in any case.

This PR only touches tests, so no changelog is warranted.

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[emolter]

regtests: https://github.com/spacetelescope/RegressionTests/actions/runs/16572916095

edit: all passing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.26%. Comparing base (0a6e722) to head (10e481c).
⚠️ Report is 287 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9706      +/-   ##
==========================================
- Coverage   81.28%   81.26%   -0.02%     
==========================================
  Files         368      368              
  Lines       37501    37501              
==========================================
- Hits        30484    30477       -7     
- Misses       7017     7024       +7     

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[melanieclarke]

LGTM, thanks for the clean up.