Add tests for docs and examples

[tlambert03]

This PR adds two more tests that make sure all of the {code-cell} blocks in the docs, and all of the *.py files in the examples folder run without errors or warnings (unless specifically expected to)

[codecov]

Codecov Report

Merging #68 (f134bfa) into master (002f973) will increase coverage by 2.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   85.07%   87.22%   +2.14%     
==========================================
  Files          15       15              
  Lines        1588     1589       +1     
==========================================
+ Hits         1351     1386      +35     
+ Misses        237      203      -34     

Impacted Files	Coverage Δ
magicgui/widgets/_concrete.py	82.67% <100.00%> (+3.07%)	⬆️
magicgui/backends/_qtpy/widgets.py	83.12% <0.00%> (+0.41%)	⬆️
magicgui/widgets/_bases.py	89.31% <0.00%> (+2.35%)	⬆️
magicgui/backends/_qtpy/application.py	91.66% <0.00%> (+2.77%)	⬆️
magicgui/type_map.py	94.73% <0.00%> (+3.00%)	⬆️
magicgui/function_gui.py	91.66% <0.00%> (+9.16%)	⬆️

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[sofroniewn]

LGTM!