Add unit test for TextBoxBaseDesigner

[Liv-Goh]

Related #10773

Proposed changes

1. Add unit test TextBoxBaseDesignerTests.cs for public properties and method of the TextBoxBaseDesigner.
2. Enable nullability in TextBoxBaseDesignerTests.cs.

Microsoft Reviewers: Open in CodeFlow

[lonitra]

Looks good, just some trailing whitespace that is causing build failure

[lonitra]

Consider doing a local build from command line to help with catching build errors that CI will produce. This can be done by running build.cmd from the command line where your local winforms repo lives.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.04403%. Comparing base (65c6beb) to head (9f8f5f3).
Report is 99 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #12433          +/-   ##
====================================================
+ Coverage   75.65340%   97.04403%   +21.39063%     
====================================================
  Files           3121        1162        -1959     
  Lines         635411      351966      -283445     
  Branches       46937        5294       -41643     
====================================================
- Hits          480710      341562      -139148     
+ Misses        151240        9658      -141582     
+ Partials        3461         746        -2715     

Flag	Coverage Δ
Debug	97.04403% <100.00000%> (+21.39063%)	⬆️
integration	?
production	?
test	97.04403% <100.00000%> (+0.01697%)	⬆️
unit	?

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