[CI] Try to fix race condition

[LennartPurucker]

Ensure we have one process per file such that autouse fixture and TestBase class do not clash.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.13%. Comparing base (d36593d) to head (ed8f4fc).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1404      +/-   ##
===========================================
+ Coverage    84.05%   84.13%   +0.07%     
===========================================
  Files           38       38              
  Lines         5230     5230              
===========================================
+ Hits          4396     4400       +4     
+ Misses         834      830       -4     

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[LennartPurucker]

Wait for #1372 to merge, rebase this, and try agian.

[LennartPurucker]

Together with the fix regarding MP spawn in #1372, this should resolve #1378

[LennartPurucker]

This does not seem to solve the problem. So we need to refactor the tests in a different way.