Add unit test for MaskedTextBoxDesignerActionList

[Syareel-Sukeri]

Related #10773

Proposed changes

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

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.83272%. Comparing base (65f897b) to head (c41aa78).
Report is 92 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12491         +/-   ##
===================================================
+ Coverage   75.65407%   75.83272%   +0.17864%     
===================================================
  Files           3150        3165         +15     
  Lines         635831      636472        +641     
  Branches       47020       47002         -18     
===================================================
+ Hits          481032      482654       +1622     
+ Misses        151352      150349       -1003     
- Partials        3447        3469         +22     

Flag	Coverage Δ
Debug	75.83272% <100.00000%> (+0.17864%)	⬆️
integration	18.16396% <ø> (-0.08512%)	⬇️
production	49.45068% <ø> (+0.25235%)	⬆️
test	97.05123% <100.00000%> (+0.02100%)	⬆️
unit	46.67078% <ø> (+0.51482%)	⬆️

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[Tanya-Solyanik]

The failing scenario could be tested as part of the MaskDesignerDialog testing, it's ok to remove it from this file.