rcal-457 Add test for spectral preview files

[ddavis-stsci]

Resolves RCAL-nnnn

Closes #

This PR adds a check for the preview file that needs to be generated for DMS278 (https://jira.stsci.edu/projects/RDM/issues/RDM-3153) and for imaging previews DMS281 (https://jira.stsci.edu/browse/RDM-3152)

Checklist

• [N/A] added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• [N/A] updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ba515d9) 71.07% compared to head (a7b44d3) 71.21%.
Report is 1 commits behind head on main.

❗ Current head a7b44d3 differs from pull request most recent head cd55252. Consider uploading reports for the commit cd55252 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1021      +/-   ##
==========================================
+ Coverage   71.07%   71.21%   +0.14%     
==========================================
  Files         104      104              
  Lines        7005     7008       +3     
==========================================
+ Hits         4979     4991      +12     
+ Misses       2026     2017       -9     

Flag	Coverage Δ		*Carryforward flag
nightly	63.29% <ø> (ø)		Carriedforward from 2f545d8

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