Add App UI testing project

We need a separate project for testing UI-related code from the App project. We currently have `Tests.App` but we probably should add `Tests.App.UI` as a separate project.

Test code needs to be launched on a UI thread inside of a window's DispatchQueue.

I've tried doing the following in a new solution to see how feasible it is to do, but have run into issue after issue:
1. create `App` project from winui3 template
1. create `AppTest` project from winui3 unit testing template
1. add `DependencyObjectSelector.cs` to `App` (which doesn't depend on anything so it's just a simple copy/paste and change namespace)
1. add `DependencyObjectSelectorTest.cs` to `AppTest`, and accept the suggestion to add a project reference to `App`
1. click the greyed out test run icon beside the test method, accept the prompt to install `Microsoft.NET.Test.Sdk`
click the green test run icon
1. `Program has more than one entry point defined. Compile with /main to specify the type that contains the entry point.`
1. add `AppTest.Program` (which is the XAML generated main from winui 3)
1. `An unhandled exception was thrown by the 'Execute' method. Please report this error to the author of the attribute 'Microsoft.VisualStudio.TestTools.UnitTesting.AppContainer.UITestMethodAttribute'.`

I've also tried using the entry point from `Microsoft.NET.Test.Sdk` (which is empty) and doing winui 3 startup in an `AssemblyInitializer`, and it gets a bit further but I run into XAML loading issues.

Start by comparing the existing Tests.App project with the proposed Tests.App.UI project, then reproduce the DependencyObjectSelectorTest.cs setup described in the issue. Read the WinUI 3 template setup and the Microsoft.NET.Test.Sdk and UITestMethodAttribute errors first. Done means a separate UI test project can launch tests on the UI thread inside a window DispatchQueue without the reported entry-point or XAML-loading failures.

Written by the indexing model from the issue text.