Add test coverage for PropertyGrid.PropertyTabCollection

[MelonWang1]

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

Proposed changes

• Add unit test for PropertyGrid.PropertyTabCollection to test its properties: Count, this.
• Add unit test for PropertyGrid.PropertyTabCollection to test its method: AddTabType.

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.36364% with 6 lines in your changes missing coverage. Please review.

Project coverage is 74.77221%. Comparing base (3f282e9) to head (0445cc9). Report is 34 commits behind head on feature/10.0.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/10.0 #11711 +/- ## ====================================================== + Coverage 74.65600% 74.77221% +0.11620% ====================================================== Files 3042 3051 +9 Lines 629873 630959 +1086 Branches 46865 46878 +13 ====================================================== + Hits 470238 471782 +1544 + Misses 156274 155817 -457 + Partials 3361 3360 -1 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11711/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/11711/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.77221% <86.36364%> (+0.11620%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11711/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.96028% <ø> (-0.01342%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11711/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.81308% <ø> (+0.15818%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11711/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.98604% <86.36364%> (+0.00769%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11711/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.85975% <ø> (+0.16174%)` | :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.