Add test coverage for NumericUpDown

[Syareel-Sukeri]

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

Proposed changes

• Add unit tests for NumericUpDown to test its properties: Hexadecimal, Increment, Maximum, Minimum, ThousandsSeparator, Value
• Add unit tests for NumericUpDown to test its events: PaddingChanged, TextChanged, ValueChanged,
• Add unit tests for NumericUpDown to test its methods: BeginInit, DownButton, EndInit, ToString, UpButton

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.05213% with 4 lines in your changes missing coverage. Please review.

Project coverage is 74.43280%. Comparing base (481bb3d) to head (16b32ff). Report is 41 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11522 +/- ## =================================================== + Coverage 74.41264% 74.43280% +0.02016% =================================================== Files 3029 3032 +3 Lines 627892 628618 +726 Branches 46770 46813 +43 =================================================== + Hits 467231 467898 +667 - Misses 157317 157361 +44 - Partials 3344 3359 +15 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11522/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/11522/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.43280% <99.05213%> (+0.02016%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11522/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.98559% <ø> (-0.01185%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11522/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.27580% <ø> (+0.03728%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11522/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.96778% <99.05213%> (-0.02191%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11522/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.26044% <ø> (+0.06471%)` | :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.