Ignore system environment variables for tests

[rytilahti]

This unsets KASA_ prefixed environment variables for cli tests, avoiding the annoying failures in auth tests if you have username and password defined in your environment.

As this is a fixture, we could potentially extend this to default catch_exceptions=False for invoke().

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (master@203bd79). Click here to learn what that means.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #851   +/-   ##
=========================================
  Coverage          ?   89.00%           
=========================================
  Files             ?       65           
  Lines             ?     4583           
  Branches          ?     1130           
=========================================
  Hits              ?     4079           
  Misses            ?      420           
  Partials          ?       84           

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[sdb9696]

I double checked we'd still be able to use isolated_filesystem etc with the fixture approach and all looks good.