Add test coverage for MaskedTextBox

[MelonWang1]

Related https://github.com/dotnet/winforms/issues/10453

Proposed changes

• Add unit tests for MaskedTextBox to test its properties: BeepOnError, AsciiOnly and Culture

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 73.20584%. Comparing base (bd97476) to head (c5922ee). Report is 45 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10978         +/-   ##
===================================================
+ Coverage   73.17333%   73.20584%   +0.03250%     
===================================================
  Files           3097        3097                 
  Lines         633955      634448        +493     
  Branches       47364       47380         +16     
===================================================
+ Hits          463886      464453        +567     
+ Misses        166522      166448         -74     
  Partials        3547        3547                 

Flag	Coverage Δ
Debug	73.20584% <100.00000%> (+0.03250%)	:arrow_up:
integration	18.44236% <ø> (-0.00840%)	:arrow_down:
production	46.71475% <ø> (+0.04067%)	:arrow_up:
test	94.97323% <100.00000%> (-0.00132%)	:arrow_down:
unit	43.61802% <ø> (+0.07276%)	:arrow_up:

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

[Tanya-Solyanik]

@LeafShi1, @SimonZhao888 , @Epica3055 , @ricardobossan - please review this PR

[ricardobossan]

LGTM