Add test coverage for ComboBox

[Nora-Zhou01]

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

Proposed changes

• Add unit tests for ComboBox to test its methods: GetItemHeight, ResetText, SelectAll

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 74.47097%. Comparing base (bde3c80) to head (8c11195). Report is 212 commits behind head on main.

:exclamation: Current head 8c11195 differs from pull request most recent head b273720

Please upload reports for the commit b273720 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11533 +/- ## =================================================== + Coverage 74.35458% 74.47097% +0.11638% =================================================== Files 3009 3032 +23 Lines 624977 628939 +3962 Branches 46497 46849 +352 =================================================== + Hits 464699 468377 +3678 - Misses 156874 157213 +339 + Partials 3404 3349 -55 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11533/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/11533/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.47097% <89.33333%> (+0.11638%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11533/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99122% <ø> (-0.37618%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11533/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.33192% <ø> (+0.36305%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11533/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.97003% <89.33333%> (-0.09511%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11533/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.31235% <ø> (+0.39968%)` | :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.

[Nora-Zhou01]

This branch has conflicts, submit a new PR to check