[MNT] WidgetTest: Cleanup on tearDown

[ales-erjavec]

Issue

Ref: biolab/orange3#7211 (comment)

Description of changes

Cleanup on tearDown. This will break at least some tests

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.05%. Comparing base (600f194) to head (d41bb7b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #290   +/-   ##
=======================================
  Coverage   72.04%   72.05%           
=======================================
  Files          34       34           
  Lines        9115     9117    +2     
=======================================
+ Hits         6567     6569    +2     
  Misses       2548     2548           

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