WPF unit test improvements for .NET 10

[JeremyKuhne]

Getting unit testing fully running is a high priority. This issue tracks the work involved.

• Get xunit working with dispatcher (DONE: Enable xunit testing in VS with the dispatcher #10023)
• Ensure that all other managed projects have matching unit test assemblies (following the pattern in Enable xunit testing in VS with the dispatcher #10023)
• Identify active and recently merged PRs that should have unit tests written.
• Create example tests that programmatically load XAML and display it
• Try porting some parts of DRT tests
• Show samples of TestAccessor to look at private members
• Document how to write tests

Will update this repeatedly as work is understood.

WinForms unit test best practices docs. Note that the docs here need to be updated. I plan to run through them soon.

@Kuldeep-MS