Ensure that all WinForms controls have unit tests coverage for public APIs

[Tanya-Solyanik]

• [x] Investigate if we can re-enable code coverage on internal builds We enabled it in #514, however Coverage option is disabled in the build pipeline - https://github.com/dotnet/winforms/blob/95e83ed6e86c93c10020a7c92a6c9d459f65f6a5/eng/pipelines/build.yml#L33.
• [x] Generate list of controls with missing test coverage for public methods
• [ ] Work with @Olina-Zhang's team on adding the missing scenarios

[paul1956]

@Olina-Zhang @Tanya-Solyanik I don't think the VB code has much test coverage. I added some in my yet to be merged PR #9867. I was not able to add anything for Network Uploads as I have no idea how to access a server to upload to.

[Tanya-Solyanik]

@paul1956 - good point about the missing VB-specific coverage. We'll add it after the C# coverage is in done.

[SimonZhao888]

Following controls are missing test coverage for public methods: System.Windows.Forms

• [ ] Button https://github.com/dotnet/winforms/pull/10693
• [ ] CheckBox https://github.com/dotnet/winforms/pull/10693, https://github.com/dotnet/winforms/pull/11249
• [ ] FlatButton https://github.com/dotnet/winforms/pull/10693
• [ ] RadioButton https://github.com/dotnet/winforms/pull/10693
• [ ] ComboBox https://github.com/dotnet/winforms/pull/11381, https://github.com/dotnet/winforms/pull/11533, https://github.com/dotnet/winforms/pull/11565
• [ ] DataGridView https://github.com/dotnet/winforms/pull/11541, https://github.com/dotnet/winforms/pull/11556
• [ ] DateTimePicker https://github.com/dotnet/winforms/pull/11033, https://github.com/dotnet/winforms/pull/11106, https://github.com/dotnet/winforms/pull/11318, https://github.com/dotnet/winforms/pull/11385, https://github.com/dotnet/winforms/pull/11534, https://github.com/dotnet/winforms/pull/11463
• [ ] GroupBox
• [ ] Label https://github.com/dotnet/winforms/pull/10787, https://github.com/dotnet/winforms/pull/10976, https://github.com/dotnet/winforms/pull/11521
• [ ] LinkLabel
• [ ] CheckedListBox https://github.com/dotnet/winforms/pull/11265, https://github.com/dotnet/winforms/pull/11317, https://github.com/dotnet/winforms/pull/11384
• [ ] ListBox
• [ ] ListView https://github.com/dotnet/winforms/pull/11264, https://github.com/dotnet/winforms/pull/11380
• [ ] MenuStrip
• [ ] MonthCalendar https://github.com/dotnet/winforms/pull/11149, https://github.com/dotnet/winforms/pull/11217, https://github.com/dotnet/winforms/pull/11519
• [ ] ProgressBar https://github.com/dotnet/winforms/pull/11074
• [ ] PropertyGrid https://github.com/dotnet/winforms/pull/11476, https://github.com/dotnet/winforms/pull/11587
• [ ] RichTextBox https://github.com/dotnet/winforms/pull/10994 https://github.com/dotnet/winforms/pull/10921 https://github.com/dotnet/winforms/pull/11034
• [ ] MaskedTextBox https://github.com/dotnet/winforms/pull/10978, https://github.com/dotnet/winforms/pull/11231, https://github.com/dotnet/winforms/pull/11286, https://github.com/dotnet/winforms/pull/11376
• [ ] TextBox https://github.com/dotnet/winforms/pull/11021
• [ ] ToolStrip https://github.com/dotnet/winforms/pull/11464
• [ ] TrackBar https://github.com/dotnet/winforms/pull/11066, https://github.com/dotnet/winforms/pull/11397
• [ ] TreeView https://github.com/dotnet/winforms/pull/11293, https://github.com/dotnet/winforms/pull/11361
• [ ] DomainUpDown https://github.com/dotnet/winforms/pull/11331
• [ ] WebBrowser https://github.com/dotnet/winforms/pull/10755
• [ ] NumericUpDown https://github.com/dotnet/winforms/pull/11522