[UIThreadTestMethod] should have Analyzer to ensure parent class inherits from VisualUITestBase

[michael-hawker]

Describe the bug

The Source Generator tries to call EnqueueAsync on the class, but this is a method from VisualUITestBase, so if you try and use the generator attribute without inheriting from the class, you can an error about it not finding the method.

We should have an analyzer here to help guide folks to knowing that they have to inherit from the class. That and/or we should figure out a way to capture the dispatcher at the right point in test initialization so that we can just use it for the generated methods without needing a special base class?

Steps to reproduce

Use `UIThreadTestMethod` in a class not inheriting from `VisualUITestBase`

Expected behavior

Help in guiding to the right path.

Screenshots

No response

Code Platform

• UWP
• WinAppSDK / WinUI 3
• Web Assembly (WASM)
• Android
• iOS
• MacOS
• Linux / GTK

Windows Build Number

• Windows 10 1809 (Build 17763)
• Windows 10 1903 (Build 18362)
• Windows 10 1909 (Build 18363)
• Windows 10 2004 (Build 19041)
• Windows 10 20H2 (Build 19042)
• Windows 10 21H1 (Build 19043)
• Windows 11 21H2 (Build 22000)
• Other (specify)

Other Windows Build number

No response

App minimum and target SDK version

• Windows 10, version 1809 (Build 17763)
• Windows 10, version 1903 (Build 18362)
• Windows 10, version 1909 (Build 18363)
• Windows 10, version 2004 (Build 19041)
• Other (specify)

Other SDK version

No response

Visual Studio Version

No response

Visual Studio Build Number

No response

Device form factor

No response

Additional context

No response