Better code coverage from tests using subprocess

[danielhollas]

In many tests we use subprocess module to ensure isolation. Collecting code coverage from such tests requires additional coveragepy configuration, see
https://coverage.readthedocs.io/en/7.11.0/config.html#run-patch

This is clearly important, as the coverage is increased by over 2% on this PR.

Follow-up to #7042.

(note: this is partially a fix for a regression from #7042. In earlier versions, the subprocess coverage was handled by pytest-cov plugin, but this support was dropped in the latest 7.0 version, in favour of a (more robust) support directly in coveragepy. See changelog for details: https://pytest-cov.readthedocs.io/en/latest/changelog.html#id1)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.63%. Comparing base (1f2cb47) to head (d73326e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7070      +/-   ##
==========================================
+ Coverage   77.60%   79.63%   +2.03%     
==========================================
  Files         566      566              
  Lines       43498    43498              
==========================================
+ Hits        33754    34635     +881     
+ Misses       9744     8863     -881     

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