With the new PlatformDispatcher.displays and FlutterView.display properties, flutter_test should have the same capability to mock and modify them as other properties on PlatformDispatcher and FlutterView.

• A new TestDisplay class will be created that allows for setting values that are read only in the Display implementation similar to TestFlutterView
  • Setters will have the same name as the get only property, not follow the old TestWindow patterns with separate names
• When no display is available (running in headless mode), a default TestDisplay will be created with properties that match up with default values for the view that would be embedded in it.
• When one or more displays are available on the PlatformDispatcher, they will be wrapped in TestDisplays similarly to TestFlutterViews
• TestFlutterView.display will reference the same TestDisplay as TestPlatformDispatcher.displays[view.displayId] so that any mocked values will be in sync regardless of how they are accessed.