Push spec coverage for Spree::Order to 100%

[mamhoff]

Summary

This adds specs for the Spree::Order model so the coverage for that file goes up to 100%. It also covers a case where Spree::Promotion is called from within the rest of the system, so adding specs here makes sure this gets picked up in #5634.

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.82%. Comparing base (928781f) to head (36d1c02).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5673      +/-   ##
==========================================
+ Coverage   88.69%   88.82%   +0.13%     
==========================================
  Files         695      695              
  Lines       16589    16589              
==========================================
+ Hits        14713    14736      +23     
+ Misses       1876     1853      -23     

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