Add unit tests for GroupBox

[Liv-Goh]

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

Proposed changes

• Add unit test for groupBox to test its property: DisplayRectangle

  Microsoft Reviewers: Open in CodeFlow

[Liv-Goh]

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[codecov[bot]]

Codecov Report

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

Project coverage is 74.65042%. Comparing base (d489380) to head (5332866). Report is 152 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11688 +/- ## =================================================== + Coverage 74.34145% 74.65042% +0.30897% =================================================== Files 3028 3042 +14 Lines 627405 629833 +2428 Branches 46770 46849 +79 =================================================== + Hits 466422 470173 +3751 + Misses 157641 156296 -1345 - Partials 3342 3364 +22 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11688/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/11688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.65042% <100.00000%> (+0.30897%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.96357% <ø> (-0.02837%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.63878% <ø> (+0.51989%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.97834% <100.00000%> (-0.00821%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.68181% <ø> (+0.61023%)` | :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.