feat: migrate to react-native-test-app

FIXME: need to ask how to customise the AppDelegate as in the previous commit, as it's complaining about:

Thread 1: "<ReactTestApp.AppDelegate: 0x10a3e7b50> does not conform to RNAppAuthAuthorizationFlowManager"

And will likely need to do similar customisation for Android.

Author: shirakaba

Diff for: examples/demo/README.md

@@ -29,11 +29,7 @@ yarn ios
 yarn android
 
 # or:
-# Run the macOS app (after setting `applePlatform = "macos"` in metro.config.js)
-#
-# Note that we do not currently support running the macOS demo simultaneously
-# with iOS or Android apps, as we use the `applePlatform = "macos"` flag to
-# force Metro to resolve react-native-macos instead of react-native.
+# Run the macOS app
 yarn macos
 ```