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 properties: AutoCompleteCustomSource, SelectedText

• Add unit test for stability and reliability of ComboBox in EndUpdate and BeginUpdate settings

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 74.29395%. Comparing base (bde3c80) to head (a3b9204). Report is 116 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11381 +/- ## =================================================== - Coverage 74.35458% 74.29395% -0.06064% =================================================== Files 3009 3026 +17 Lines 624977 627152 +2175 Branches 46497 46758 +261 =================================================== + Hits 464699 465936 +1237 - Misses 156874 157870 +996 + Partials 3404 3346 -58 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11381/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/11381/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.29395% <100.00000%> (-0.06064%)` | :arrow_down: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11381/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99823% <ø> (-0.36917%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11381/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.03027% <ø> (+0.06141%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11381/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.98768% <100.00000%> (-0.07745%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11381/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.00946% <ø> (+0.09678%)` | :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.