Extract the app and abstract the testing framework and add NUnit

[mattleibow]

This PR will reconfigure all the code to allow for more than just Xunit as the testing framework. The current implementation is very tied to Xunit. I previously abstracted the testing later from Maui to allow for future native apps.

This PR now goes one step further and abstracts all the things so we can have:

• [x] .NET MAUI testing app
• [x] Xunit tests
• [x] NUnit tests
• [x] Headless Xunit tests with XHarness

The future could bring things like:

• MSTest tests
• Headless NUnit tests with XHarness
• Native testing apps

[MartyIX]

Is it xUnit instead of NUnit?

[mattleibow]

Is it xUnit instead of NUnit?

It will be BOTH! 😀