JP-624: Add unit tests to skymatch

[mcara]

Resolves JP-624

This PR addresses adds unit tests for the skymatch step

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.23 🎉

Comparison is base (33f102e) 77.62% compared to head (8737b6b) 77.86%.

❗ Current head 8737b6b differs from pull request most recent head 68944aa. Consider uploading reports for the commit 68944aa to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7531      +/-   ##
==========================================
+ Coverage   77.62%   77.86%   +0.23%     
==========================================
  Files         452      452              
  Lines       36206    36206              
==========================================
+ Hits        28105    28191      +86     
+ Misses       8101     8015      -86     

Flag	Coverage Δ		*Carryforward flag
nightly	77.64% <ø> (ø)		Carriedforward from d61e0b1

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
jwst/skymatch/skymatch_step.py	92.22% <100.00%> (+21.11%)	⬆️

... and 4 files with indirect coverage changes

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[hbushouse]

LGTM

[mcara]

@hbushouse Please do not merge yet. I plan to add tests for the reduced memory usage. I think this would be useful (in the future).

[nden]

lgtm

[mcara]

The average coverage for the skymatch step is 80% with most files being covered > 90%