Add test coverage for PropertyGrid

[MelonWang1]

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

Proposed changes

• Add unit tests for PropertyGrid to test its events: PropertyValueChanged, PropertySortChanged, SelectedObjectsChanged.

• Add unit tests for PropertyGrid to test its methods: CollapseAndExpandAllGridItems, ResetSelectedProperty.

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 74.40719%. Comparing base (c3dd6f4) to head (755568f). Report is 51 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11476 +/- ## =================================================== + Coverage 74.36489% 74.40719% +0.04229% =================================================== Files 3028 3032 +4 Lines 627534 628157 +623 Branches 46764 46811 +47 =================================================== + Hits 466665 467394 +729 + Misses 157529 157415 -114 - Partials 3340 3348 +8 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11476/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/11476/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.40719% <100.00000%> (+0.04229%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11476/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99332% <ø> (+0.00046%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11476/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.25447% <ø> (+0.08930%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11476/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.96730% <100.00000%> (-0.01955%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11476/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.23548% <ø> (+0.11871%)` | :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.