Fix tests on wx

[gselzer]

This PR adds some test code to clean up Wx windows before closing out, solving some CI failures (and local warnings) noticed first in #283

Unfortunately this currently causes widget leaks in qt :(

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.96%. Comparing base (0f1407f) to head (3933405).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #284       +/-   ##
===========================================
- Coverage   85.40%   72.96%   -12.44%     
===========================================
  Files          53       53               
  Lines        7192     7192               
===========================================
- Hits         6142     5248      -894     
- Misses       1050     1944      +894     

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