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]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 canceled.

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🔨 Latest commit	a838060
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.09%. Comparing base (aaea93a) to head (16f78b7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1147      +/-   ##
==========================================
+ Coverage   83.29%   84.09%   +0.79%     
==========================================
  Files          59       59              
  Lines        2449     2452       +3     
  Branches      280      280              
==========================================
+ Hits         2040     2062      +22     
+ Misses        365      345      -20     
- Partials       44       45       +1     

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