Use same local Dask cluster in tests

[flying-sheep]

Since #1906, tests often fail because multiple workers want to spin up a local cluster.

This could be fixed using processes=False clusters or by reusing a cluster.

TODO:

• Tell people here once my account is activated: https://dask.discourse.group/t/using-pytest-with-localclusters/1053/2

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.37%. Comparing base (762094b) to head (5ac05aa).
Report is 48 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1924      +/-   ##
==========================================
+ Coverage   82.82%   84.37%   +1.54%     
==========================================
  Files          45       45              
  Lines        6777     6777              
==========================================
+ Hits         5613     5718     +105     
+ Misses       1164     1059     -105     

see 15 files with indirect coverage changes

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[flying-sheep]

OK, I have to merge this, GitHub behaves weird in other jobs and this might help.