Load testhelpers into test modules instead of Main

[jishnub]

This avoids unnecessary namespace collisions in Main.

[KristofferC]

But now you are including the same file many times and you will compile the code in those files multiple times (instead of once per process as before)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.81%. Comparing base (b265fea) to head (14e0442).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1376   +/-   ##
=======================================
  Coverage   93.81%   93.81%           
=======================================
  Files          34       34           
  Lines       15717    15717           
=======================================
  Hits        14745    14745           
  Misses        972      972           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jishnub]

I've changed this to include a single file into Main that defines a test-helpers module, and individual test files import submodules from this. This achieves the purpose of avoiding namespace collisions to a large extent.