Add code coverage during CI testing

[sloede]

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[sloede]

@simonechiocchetti This adds standard code coverage checking for the tests to see which lines of the code are actually used during CI testing.

[sloede]

Note that once this is merged, I will need to update the required checks (because the names have changed)

[sloede]

Note that once this is merged, I will need to update the required checks (because the names have changed)

(that's also the reason why auto-merge will not work here)