Add test coverage for MaskedTextBox

[MelonWang1]

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

Proposed changes

• Add unit tests for MaskedTextBox to test its properties: MaskedTextBox_RejectInputOnFirstFailure, ResetOnPrompt, ValidatingType, PromptChar, PasswordChar.
• Add unit tests for MaskedTextBox to test its events: TypeValidationCompletedEvent.

• Add unit tests for MaskedTextBox to test its methods: GetCharFromPosition, GetCharIndexFromPosition, GetPositionFromCharIndex, ValidateText.

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 74.31702%. Comparing base (f2118b8) to head (56e598f). Report is 33 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11376 +/- ## =================================================== + Coverage 74.27372% 74.31702% +0.04330% =================================================== Files 3025 3026 +1 Lines 626861 627248 +387 Branches 46742 46760 +18 =================================================== + Hits 465593 466152 +559 + Misses 157926 157751 -175 - Partials 3342 3345 +3 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/11376/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/11376/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `74.31702% <100.00000%> (+0.04330%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/11376/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99788% <ø> (+0.00194%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/11376/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `47.07239% <ø> (+0.07069%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/11376/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `96.98852% <100.00000%> (-0.00564%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/11376/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `44.04772% <ø> (+0.07663%)` | :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.

[MelonWang1]

/azp run

[azure-pipelines[bot]]

Commenter does not have sufficient privileges for PR 11376 in repo dotnet/winforms