Add unit tests for ToolStripContentPanelDesigner

[Syareel-Sukeri]

Related https://github.com/dotnet/winforms/issues/10773

Proposed changes

• Add unit test ToolStripContentPanelDesignerTests.cs for public properties and method of the ToolStripContentPanelDesigner.
• Enable nullability in ToolStripContentPanelDesignerTests.cs.

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.58677%. Comparing base (89e6bb9) to head (b412578). Report is 12 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12366         +/-   ##
===================================================
- Coverage   75.64669%   75.58677%   -0.05993%     
===================================================
  Files           3119        3136         +17     
  Lines         635392      636020        +628     
  Branches       46933       47025         +92     
===================================================
+ Hits          480653      480747         +94     
- Misses        151277      151813        +536     
+ Partials        3462        3460          -2     

Flag	Coverage Δ
Debug	75.58677% <100.00000%> (-0.05993%)	:arrow_down:
integration	18.24448% <ø> (-0.03792%)	:arrow_down:
production	49.11566% <ø> (-0.08446%)	:arrow_down:
test	97.02732% <100.00000%> (+0.00042%)	:arrow_up:
unit	46.09831% <ø> (-0.07304%)	:arrow_down:

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