Add test coverage for TreeView

[Zheng-Li01]

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

Proposed changes

• Add unit tests for TreeView to test its properties:TopNode, accessibilityObject, ItemHeight,Indent
• Add unit tests for TreeView to test its methods: ConvertTo, CollapseAll, ExpandAll, GetNodeCount, ShouldSerializeIndent, shouldSerializeItemHeightMethod, ToString

[codecov[bot]]

Codecov Report

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

Project coverage is 74.28680%. Comparing base (9f8f671) to head (2f8afe1). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11361 +/- ## =================================================== + Coverage 74.27085% 74.28680% +0.01595% =================================================== Files 3025 3026 +1 Lines 626861 626966 +105 Branches 46742 46753 +11 =================================================== + Hits 465575 465753 +178 + Misses 157937 157868 -69 + Partials 3349 3345 -4 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11361/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/11361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.28680% <100.00000%> (+0.01595%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `18.00648% <ø> (+0.01476%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.02100% <ø> (+0.02562%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.99508% <100.00000%> (+0.00092%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11361/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `43.99110% <ø> (+0.02106%)` | :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]

@Zheng-Li01 - feel free to use only the Proposed changes section in the PR description, the rest is not applicable to new test cases

[Epica3055]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[LeafShi1]

LGTM! @Tanya-Solyanik Please review this again