Add unit test for DateTimePicker 8 events

[John-Qiao]

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

Proposed changes

• Add unit tests for below 8 events

1. FormatChanged
2. Paint
3. MouseClick
4. MouseDoubleClick
5. CloseUp
6. RightToLeftLayoutChanged
7. ValueChanged

8. DropDown

   Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 74.37873%. Comparing base (c3dd6f4) to head (2fa25fc). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11463 +/- ## =================================================== + Coverage 74.36489% 74.37873% +0.01384% =================================================== Files 3028 3028 Lines 627534 627670 +136 Branches 46764 46764 =================================================== + Hits 466665 466853 +188 + Misses 157529 157478 -51 + Partials 3340 3339 -1 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11463/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/11463/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.37873% <100.00000%> (+0.01384%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11463/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99426% <ø> (+0.00140%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11463/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.18341% <ø> (+0.01825%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11463/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.98803% <100.00000%> (+0.00118%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11463/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.13467% <ø> (+0.01789%)` | :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.