WPF unit test improvements for .NET 10

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

- [x] Get xunit working with dispatcher (DONE: #10023)
- [ ] Ensure that all other managed projects have matching unit test assemblies (following the pattern in #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](https://github.com/dotnet/winforms/blob/main/docs/testing.md#unit-test-best-practices). Note that the docs here need to be updated. I plan to run through them soon.

@Kuldeep-MS