chore(api): remove test-only function from production helper file

[matt-codecov]

needed for #188

apps/codecov-api/billing/helpers.py is imported in prod, but one of the things in it is a test-only helper that referenced test-only deps. if you start the service without dev dependencies installed, it blows up when importing that file.

so, this PR moves the test-only stuff to a different file that is not included by prod code

Stack info:

Stack 1
build: unified Dockerfile #186 ➡️chore(api): remove test-only function from production helper file #187 build: trigger all tests when reqs and build glue are changed #190 build: remove dev deps from prod image, add separate test-reqs and test-app images #188

[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.21%. Comparing base (d3d8f72) to head (78ca2a7).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files        1214     1215    +1     
  Lines       45022    45023    +1     
  Branches     1435     1435           
=======================================
+ Hits        42416    42417    +1     
  Misses       2305     2305           
  Partials      301      301           

Flag	Coverage Δ
apiunit	96.47% <100.00%> (+<0.01%)	⬆️

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