Add test coverage for LinkLabel.LinkCollection

[Philip-Wang01]

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

Proposed changes

• Add unit tests for LinkLabel.LinkCollection to test its properties: IsReadOnly, IndexOf, IndexOfKey

• Add unit tests for LinkLabel.LinkCollection to test its method: Add, Contains, ContainsKey, Remove, RemoveByKey

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 74.30362%. Comparing base (9f8f671) to head (fe3714d). Report is 34 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11357         +/-   ##
===================================================
+ Coverage   74.27085%   74.30362%   +0.03276%     
===================================================
  Files           3025        3027          +2     
  Lines         626861      627135        +274     
  Branches       46742       46756         +14     
===================================================
+ Hits          465575      465984        +409     
+ Misses        157937      157809        -128     
+ Partials        3349        3342          -7     

Flag	Coverage Δ
Debug	74.30362% <100.00000%> (+0.03276%)	:arrow_up:
integration	18.00034% <ø> (+0.00863%)	:arrow_up:
production	47.05308% <ø> (+0.05771%)	:arrow_up:
test	96.98753% <100.00000%> (-0.00662%)	:arrow_down:
unit	44.00315% <ø> (+0.03311%)	:arrow_up:

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

[Zheng-Li01]

For new added LinkCollectionTest.cs file, always named it to LinkCollectionTests.cs