TST: Fix plot composition test error messages

[has2k1]

There are two issues:

1. We had not "opted" into the custom error messaging for the tests.
2. We have to be careful that we attach the result from compare_image to the original Compare instance (cmp._err) and not a copy of it.

closes #955

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.72%. Comparing base (7c5005f) to head (9444fef).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #956   +/-   ##
=======================================
  Coverage   86.72%   86.72%           
=======================================
  Files         194      194           
  Lines       13321    13321           
  Branches     1639     1639           
=======================================
  Hits        11553    11553           
  Misses       1222     1222           
  Partials      546      546           

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