test: more test coverage, organization, and documentation

[tlambert03]

No description provided.

[codecov]

Codecov Report

Merging #7 (5261296) into main (24fe82c) will increase coverage by 6.65%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   92.58%   99.24%   +6.65%     
==========================================
  Files          17       17              
  Lines         647      658      +11     
==========================================
+ Hits          599      653      +54     
+ Misses         48        5      -43     

Impacted Files	Coverage Δ
src/app_model/expressions/__init__.py	100.00% <ø> (ø)
src/app_model/expressions/_context.py	100.00% <ø> (+5.00%)	⬆️
src/app_model/registries/__init__.py	100.00% <ø> (ø)
src/app_model/registries/_commands.py	93.22% <ø> (+11.55%)	⬆️
src/app_model/registries/_keybindings.py	100.00% <ø> (+6.45%)	⬆️
src/app_model/types/__init__.py	100.00% <ø> (ø)
src/app_model/types/_base.py	100.00% <ø> (ø)
src/app_model/expressions/_expressions.py	100.00% <100.00%> (+3.04%)	⬆️
src/app_model/registries/_menus.py	100.00% <100.00%> (+34.84%)	⬆️
src/app_model/types/_command.py	100.00% <100.00%> (ø)
... and 8 more

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