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

```diff @@ 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](https://app.codecov.io/gh/dotnet/winforms/pull/11357/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | Coverage Δ | | |---|---|---| | [Debug](https://app.codecov.io/gh/dotnet/winforms/pull/11357/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.30362% <100.00000%> (+0.03276%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11357/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `18.00034% <ø> (+0.00863%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11357/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.05308% <ø> (+0.05771%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11357/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.98753% <100.00000%> (-0.00662%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11357/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.00315% <ø> (+0.03311%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet#carryforward-flags-in-the-pull-request-comment) to find out more.

[Zheng-Li01]

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