[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.