docs: testing basics tutorial

[mdjastrzebski]

Summary

Test plan

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.38%. Comparing base (b948dce) to head (367787f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1789   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files          94       94           
  Lines        5244     5244           
  Branches      892      892           
=======================================
  Hits         5002     5002           
  Misses        242      242           

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