Move shared examples for promo calculators to lib/

[mamhoff]

Summary

This way, the shared examples can be extended and used in consuming apps.

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.44%. Comparing base (3660447) to head (99bf07f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6367   +/-   ##
=======================================
  Coverage   89.44%   89.44%           
=======================================
  Files         972      973    +1     
  Lines       20297    20303    +6     
=======================================
+ Hits        18155    18161    +6     
  Misses       2142     2142           

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[tvdeyen]

Yeah. Thanks