Codecov: collect test analytics

[wil93]

https://docs.codecov.com/docs/test-analytics#failed-test-reporting

This enables Codecov to report failed tests directly in the PR, avoiding us from having to open the github action to see what failed (at least for unit tests).

For an example, see edit history of this comment: wil93#8 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.01%. Comparing base (b77c87b) to head (ebc344a).
Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1281      +/-   ##
==========================================
- Coverage   69.39%   69.01%   -0.38%     
==========================================
  Files         328      329       +1     
  Lines       26201    26152      -49     
==========================================
- Hits        18182    18050     -132     
- Misses       8019     8102      +83     

Flag	Coverage Δ
functionaltests	46.96% <ø> (+0.20%)	⬆️
unittests	56.34% <ø> (-0.46%)	⬇️

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