Unit test source catalog against specific values

[emolter]

Relates to #9671

This PR adds a unit test to the source catalog step, which was originally written in support of the above-linked PR. That PR is getting closed without merge but the unit test is still useful to have.

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

[melanieclarke]

Always in favor of more tests!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.44%. Comparing base (076c30b) to head (9795441).
⚠️ Report is 292 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9691   +/-   ##
=======================================
  Coverage   80.44%   80.44%           
=======================================
  Files         367      367           
  Lines       37407    37407           
=======================================
  Hits        30093    30093           
  Misses       7314     7314           

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