Add xUnit Avalonia.Headless sample for e2e testing

[maxkatz6]

Topics to cover

Based on https://docs.avaloniaui.net/docs/next/concepts/headless/

Ideas for a sample

Unit test sample where:

• open a window with a viewmodel
• find a textbox
• send TextInput event
• find a button
• send a click/press event
• check that viewmodel got command with text executed

[timunie]

good sample idea as well. Unit tests get way too less attention.