chore: add testing documentation and coverage checks

[jescalada]

Implements the coverage checks mentioned in #1019, although it doesn't add any auto-merge on coverage attained (we still want maintainers to take a look at larger PRs and decide the right time to merge).

I've also added some documentation on how to implement unit, E2E and fuzz tests for new PRs.

[netlify]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.22%. Comparing base (1232fa5) to head (0ffb79c).
⚠️ Report is 30 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1147      +/-   ##
==========================================
+ Coverage   82.86%   83.22%   +0.35%     
==========================================
  Files          66       66              
  Lines        2795     2795              
  Branches      332      332              
==========================================
+ Hits         2316     2326      +10     
+ Misses        431      423       -8     
+ Partials       48       46       -2     

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[dcoric]

Very good in general, I just added a small nitpick.

[kriswest]

Lots of good stuff here! I'd like to see resetting stubs emphasized as I lost a couple of hours to debugging confusing issues caused by stubs that were not reset.

[kriswest]

Might be worth updating the template test and including sinon/proxyquire examples in it: https://github.com/finos/git-proxy/blob/main/test/1.test.js

[jescalada]

@kriswest Should be ready for a final look!

[kriswest]

LGTM