Use unittest.mock.patch for all os.environ tests

[pentschev]

Makes tests depending on os.environ settings less error-prone by applying changes via unittest.mock.patch that are automatically cleaned up.

[madsbk]

LGTM

[codecov-commenter]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.12     #787   +/-   ##
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  Coverage                ?   90.16%           
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  Files                   ?       15           
  Lines                   ?     1992           
  Branches                ?        0           
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  Hits                    ?     1796           
  Misses                  ?      196           
  Partials                ?        0           

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[pentschev]

Thanks @madsbk for the review!

[pentschev]

@gpucibot merge