test: [wip] mock tests

[tlambert03]

will close #508

the goal here is to test magicgui without any backend involvement. indirectly inspired by #505 ... where it would have been nice to immediately know if this was a backend implementation vs magicgui problem

[codecov]

Codecov Report

Base: 89.61% // Head: 89.73% // Increases project coverage by +0.12% 🎉

Coverage data is based on head (d2e39dc) compared to base (376933a).
Patch coverage: 73.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #510      +/-   ##
==========================================
+ Coverage   89.61%   89.73%   +0.12%     
==========================================
  Files          36       36              
  Lines        4353     4356       +3     
==========================================
+ Hits         3901     3909       +8     
+ Misses        452      447       -5     

Impacted Files	Coverage Δ
src/magicgui/widgets/bases/_value_widget.py	98.33% <ø> (ø)
src/magicgui/application.py	83.69% <60.00%> (+0.54%)	⬆️
src/magicgui/tqdm.py	94.59% <100.00%> (ø)
src/magicgui/widgets/bases/_widget.py	90.45% <100.00%> (+2.51%)	⬆️

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