Add code 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

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

Project coverage is 74.48360%. Comparing base (566bec4) to head (7cece47). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11565 +/- ## =================================================== - Coverage 74.48439% 74.48360% -0.00080% =================================================== Files 3039 3039 Lines 629066 629121 +55 Branches 46834 46836 +2 =================================================== + Hits 468556 468592 +36 - Misses 157154 157179 +25 + Partials 3356 3350 -6 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11565/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/11565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.48360% <100.00000%> (-0.00080%)` | :arrow_down: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.98018% <ø> (+0.00168%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.36260% <ø> (+0.01429%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.95292% <100.00000%> (-0.01832%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.32283% <ø> (-0.01528%)` | :arrow_down: | 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.

[LeafShi1]

@lonitra Please help review this PR