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

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.73%. Comparing base (376933a) to head (d2e39dc).
⚠️ Report is 145 commits behind head on main.

Files with missing lines	Patch %	Lines
src/magicgui/application.py	60.00%	4 Missing ⚠️

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     

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