Improve test coverage

[tlambert03]

Just regaining some of the coverage that was lost in #43...
there are two or three very small bugs that are fixed here as well

[codecov]

Codecov Report

Merging #70 (e88c4d3) into master (c441861) will increase coverage by 7.68%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   77.39%   85.07%   +7.68%     
==========================================
  Files          16       15       -1     
  Lines        1964     1588     -376     
==========================================
- Hits         1520     1351     -169     
+ Misses        444      237     -207     

Impacted Files	Coverage Δ
magicgui/widgets/_bases.py	86.96% <85.71%> (+10.09%)	⬆️
magicgui/backends/_qtpy/widgets.py	82.71% <100.00%> (+4.20%)	⬆️
magicgui/type_map.py	91.72% <100.00%> (+0.06%)	⬆️
magicgui/_qt/widgets.py	0.00% <0.00%> (ø)
magicgui/events.py
magicgui/function_gui.py	82.50% <0.00%> (+1.19%)	⬆️
magicgui/widgets/_concrete.py	79.60% <0.00%> (+2.41%)	⬆️
magicgui/backends/_qtpy/application.py	88.88% <0.00%> (+2.77%)	⬆️
... and 1 more

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