Add unit tests for DomainUpDown

[Syareel-Sukeri]

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

Proposed changes

• Add unit tests for DomainUpDown to test DownButton and UpButton methods: Verify the behavior of the DownButton and UpButton methods in the DomainUpDown class and how they change the SelectedIndex under different conditions
• Add unit test for DomainUpDown to test ToString method: Test ToString method's output, ensuring it correctly reflects the state of the Items collection and the SelectedIndex.

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.37107% with 5 lines in your changes missing coverage. Please review.

Project coverage is 74.38096%. Comparing base (c4596e2) to head (4d8d82c). Report is 83 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11331         +/-   ##
===================================================
+ Coverage   74.22320%   74.38096%   +0.15775%     
===================================================
  Files           3021        3028          +7     
  Lines         626156      627666       +1510     
  Branches       46698       46764         +66     
===================================================
+ Hits          464753      466864       +2111     
+ Misses        158006      157453        -553     
+ Partials        3397        3349         -48     

Flag	Coverage Δ
Debug	74.38096% <99.37107%> (+0.15775%)	:arrow_up:
integration	17.98687% <ø> (-0.28269%)	:arrow_down:
production	47.18868% <ø> (+0.30856%)	:arrow_up:
test	96.98800% <99.37107%> (-0.05071%)	:arrow_down:
unit	44.16941% <ø> (+0.33545%)	:arrow_up:

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[LeafShi1]

LGTM! @lonitra Please review this PR.