Add unit tests for DomainUpDown

[Syareel-Sukeri]

related #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]

Codecov Report

Attention: Patch coverage is 99.42857% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 74.27404%. Comparing base (c4596e2) to head (30bd5c7).
Report is 47 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11331         +/-   ##
===================================================
+ Coverage   74.22320%   74.27404%   +0.05083%     
===================================================
  Files           3021        3025          +4     
  Lines         626156      626857        +701     
  Branches       46698       46742         +44     
===================================================
+ Hits          464753      465592        +839     
+ Misses        158006      157920         -86     
+ Partials        3397        3345         -52     

Flag	Coverage Δ
Debug	74.27404% <99.42857%> (+0.05083%)	⬆️
integration	18.00578% <ø> (-0.26378%)	⬇️
production	47.00274% <ø> (+0.12263%)	⬆️
test	96.99412% <99.42857%> (-0.04459%)	⬇️
unit	43.97179% <ø> (+0.13783%)	⬆️

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[LeafShi1]

LGTM! @lonitra Please review this PR.

[Tanya-Solyanik]

Please use fluent assertions.