Calculator spec: Fix Rubocop offenses

[mamhoff]

Summary

Moves some classes to become anonymous classes. Fixes a Rubocop offense. Extracted from #6287

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.95%. Comparing base (a2a55d6) to head (80fd096).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6289   +/-   ##
=======================================
  Coverage   88.95%   88.95%           
=======================================
  Files         860      860           
  Lines       18423    18423           
=======================================
  Hits        16388    16388           
  Misses       2035     2035           

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[tvdeyen]

Needs a rebase