Improve test coverage for Spree::Adjustment to 100%

[mamhoff]

Summary

This tests a few previously untested methods on the Spree::Adjustment model. I'm doing this because the work in #5634 removes some lines from the file, leading to a negative change in test coverage because the overall file gets smaller. By increasing coverage to 100%, a smaller file will not be punished with a larger proportion of untested lines.

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.72%. Comparing base (928781f) to head (14531c0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5672      +/-   ##
==========================================
+ Coverage   88.69%   88.72%   +0.03%     
==========================================
  Files         695      695              
  Lines       16589    16589              
==========================================
+ Hits        14713    14718       +5     
+ Misses       1876     1871       -5     

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[tvdeyen]

Thanks 😊