Add unit tests for ToolStripContainerDesigner

[Olina-Zhang]

Related https://github.com/dotnet/winforms/issues/10773

Proposed changes

• Add unit test ToolStripContainerDesignerTests.cs for public properties and method of the ToolStripContainerDesigner.

• Enable nullability in ToolStripContainerDesignerTests.cs.

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.82759% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.46095%. Comparing base (99d42a9) to head (3d1888e). Report is 68 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12200 +/- ## =================================================== + Coverage 75.26640% 75.46095% +0.19454% =================================================== Files 3086 3106 +20 Lines 633636 634401 +765 Branches 46839 46879 +40 =================================================== + Hits 476915 478725 +1810 + Misses 153316 152252 -1064 - Partials 3405 3424 +19 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/12200/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/12200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.46095% <94.82759%> (+0.19454%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/12200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.98449% <ø> (-0.03683%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/12200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `48.86954% <ø> (+0.29711%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/12200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `97.02524% <94.82759%> (+0.00121%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/12200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `45.88172% <ø> (+0.27266%)` | :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.