Add pytest-xdist and pytest-randomly to run tests in random order

[hoxbro]

Built on top of #489

[codecov-commenter]

Codecov Report

Merging #492 (d9833c3) into main (592e75b) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
+ Coverage   69.36%   69.39%   +0.02%     
==========================================
  Files          79       79              
  Lines        9128     9129       +1     
==========================================
+ Hits         6332     6335       +3     
+ Misses       2796     2794       -2     

Files	Coverage Δ
lumen/tests/sources/test_duckdb.py	97.36% <100.00%> (+0.03%)	:arrow_up:

... and 2 files with indirect coverage changes

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