Add code coverage for buttons

[ricardobossan]

related #10453
related #10773

Proposed changes

Add unit tests for FlatButton, RadioButton, CheckBox, RadioButtonRenderer and CheckBoxRenderer.

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Test environment(s)

• 9.0.100-alpha.1.23618.3

Microsoft Reviewers: Open in CodeFlow

[ricardobossan]

Drafted because there's more code to add coverage to.

[codecov]

Codecov Report

Attention: Patch coverage is 92.74194% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 73.36648%. Comparing base (5c38a91) to head (656d3c9).
Report is 7 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10693         +/-   ##
===================================================
+ Coverage   73.32489%   73.36648%   +0.04158%     
===================================================
  Files           3094        3099          +5     
  Lines         632406      632804        +398     
  Branches       46857       46880         +23     
===================================================
+ Hits          463711      464266        +555     
+ Misses        165250      165079        -171     
- Partials        3445        3459         +14     

Flag	Coverage Δ
Debug	73.36648% <92.74194%> (+0.04158%)	⬆️
integration	18.29153% <ø> (-0.07114%)	⬇️
production	46.81503% <ø> (+0.04970%)	⬆️
test	94.98357% <92.74194%> (+0.00018%)	⬆️
unit	43.85612% <ø> (+0.09434%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[Tanya-Solyanik]

Great new tests! I added some comments that apply to multiple spots, but I had not repeated them everywhere, please review your changes with these comments in mind.

It will be outside of scope for this PR to move all tests to a new model where test is defined in the base class, but the Fact is implemented in a derived class. But you could implement new tests so that they follow this pattern.

Test naming is clarified here - https://github.com/dotnet/winforms/blob/main/docs/testing.md#unit-test-best-practices

[Tanya-Solyanik]

Please rebase on the latest.