Add test coverage for CheckedListBox.CheckedItemCollection and CheckedIndexCollection

[Olina-Zhang]

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

Proposed changes

• Add unit tests for CheckedListBox.CheckedItemCollection to test its properties: SyncRoot, IsSynchronized, IsFixedSize, IsReadOnly
• Add unit tests for CheckedListBox.CheckedItemCollection to test its methods: Add, Clear, Insert, Remove, RemoveAt, Contains, IndexOf, CopyTo, GetEnumerator

• Update unit tests for CheckedListBox.CheckedIndexCollection to test its methods: Contains, IndexOf

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 74.29662%. Comparing base (f2118b8) to head (1c83a2a). Report is 33 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11384 +/- ## =================================================== + Coverage 74.27372% 74.29662% +0.02290% =================================================== Files 3025 3027 +2 Lines 626861 627151 +290 Branches 46742 46761 +19 =================================================== + Hits 465593 465952 +359 + Misses 157926 157852 -74 - Partials 3342 3347 +5 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11384/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/11384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.29662% <100.00000%> (+0.02290%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99717% <ø> (+0.00123%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.03623% <ø> (+0.03453%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.98767% <100.00000%> (-0.00649%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.01403% <ø> (+0.04294%)` | :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.