Add code coverage for ToolStripSplitButton

[Zheng-Li01]

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

Proposed changes Add unit tests for ToolStripSplitButton.cs to test its properties & methods & events

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 75.46470%. Comparing base (0d0e1c9) to head (5860bbb). Report is 44 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12238 +/- ## =================================================== + Coverage 75.42712% 75.46470% +0.03757% =================================================== Files 3102 3105 +3 Lines 634244 634551 +307 Branches 46866 46889 +23 =================================================== + Hits 478392 478862 +470 + Misses 152434 152264 -170 - Partials 3418 3425 +7 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/12238/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/12238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.46470% <100.00000%> (+0.03757%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/12238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.97708% <ø> (-0.00250%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/12238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `48.86426% <ø> (+0.06056%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/12238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `97.02708% <100.00000%> (+0.00083%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/12238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `45.87257% <ø> (+0.03670%)` | :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.

[Tanya-Solyanik]

@LeafShi1 - please re-review