Migrate testing_app to UIScene lifecycle (#2836)

## Summary
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- Migrate `testing_app` iOS project to the UIScene lifecycle as per the
[migration
guide](https://docs.flutter.dev/release/breaking-changes/uiscenedelegate)
- Update `AppDelegate.swift` to use `@main`,
`FlutterImplicitEngineDelegate`, and move plugin registration to
`didInitializeImplicitFlutterEngine`
- Add `UIApplicationSceneManifest` to `Info.plist`

Part of flutter/flutter#176957

## Test plan
- [x] Ran `testing_app` on iOS simulator — app launches and works
correctly

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

pubspec.yaml

@@ -50,8 +50,8 @@ skip_ci:
   - add_to_app/multiple_flutters/multiple_flutters_module
   - add_to_app/plugin/flutter_module_using_plugin
   - add_to_app/prebuilt_module/flutter_module
-  # remove after 3.44 release
   - cupertino_gallery
 
 skip_ci_beta:
-  - material_3_demo
+  # - sample_name_to_skip
+