Add a new Montage accuracy test in ICRS

[astrofrog]

Inaccuracies in the current Montage comparison test are due to different assumptions for the Equatorial <-> Galactic conversion. This adds a test in ICRS to make sure are actually comparing the pixel overlap calculations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.07%. Comparing base (64ffcea) to head (702c804).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #590   +/-   ##
=======================================
  Coverage   91.06%   91.07%           
=======================================
  Files          30       30           
  Lines        1858     1859    +1     
=======================================
+ Hits         1692     1693    +1     
  Misses        166      166           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.