Copy assertion for mode equality from asdf-astropy

[nden]

This PR copies asdf-astropy.testing.helpers.assert_model_equal to roman_datamodels.testing.assertions.
The testing functions in roman_datamodels are used by CRDS tests. However, CRDS tests do not use pytest and CRDS has no dependency on pytest. Importing from asdf-astropy adds pytest as a dependency of CRDS which is undesirable.

Alternatively this can be fixed in asdf-astropy in a future release because the pytest dependency is coming from an import from astropy.units which can be done within a function. However, in the interest of releasing this for build 6, the function is copied now here.

[codecov]

Codecov Report

Merging #79 (160d989) into main (9ef4230) will increase coverage by 0.37%.
The diff coverage is 100.00%.

❗ Current head 160d989 differs from pull request most recent head 543955b. Consider uploading reports for the commit 543955b to get more accurate results

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   66.07%   66.45%   +0.37%     
==========================================
  Files          15       15              
  Lines        1981     2003      +22     
==========================================
+ Hits         1309     1331      +22     
  Misses        672      672              

Impacted Files	Coverage Δ
src/roman_datamodels/testing/assertions.py	97.95% <100.00%> (+1.66%)	⬆️

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[alphasentaurii]

LGTM - thanks!