RNTesterIntegrationTests can fail due to the warning "Can't call setState (or forceUpdate) on an unmounted component." (react#24984)

Summary:
When running the RNTesterIntegrationTests from the XCode IDE or xcodebuild command line, its possible for tests to intermittently fail due to the warning `'Warning: Can\'t call setState (or forceUpdate) on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.%s', '\n in RNTesterApp.` .

A setState() call could happen in the AsyncStorage callback after the component had unmounted: presumably because the test ran and concluded before AsyncStorage returned. Changed RNTesterApp.ios.js to maintain a _mounted field that is set and cleared in componentDidMount() and componentWillUnmount() and refrain from calling setState() if the flag is false.

## Changelog

[iOS] [Fixed] - Fixed RNTesterApp to not call setState() after the component has been unmounted.
Pull Request resolved: react#24984

Differential Revision: D15447898

Pulled By: hramos

fbshipit-source-id: b01bc0a40a4f4d548aca0a4bb891ba3f4c8d0612

Author: tom-un

RNTester/js/RNTesterApp.ios.js

@@ -61,13 +61,19 @@ const Header = ({onBack, title}: {onBack?: () => mixed, title: string}) => (
 );
 
 class RNTesterApp extends React.Component<Props, RNTesterNavigationState> {
+  _mounted: boolean;
+
   UNSAFE_componentWillMount() {
     BackHandler.addEventListener('hardwareBackPress', this._handleBack);
   }
 
   componentDidMount() {
+    this._mounted = true;
     Linking.getInitialURL().then(url => {
       AsyncStorage.getItem(APP_STATE_KEY, (err, storedString) => {
+        if (!this._mounted) {
+          return;
+        }
         const exampleAction = URIActionMap(
           this.props.exampleFromAppetizeParams,
         );
@@ -83,6 +89,10 @@ class RNTesterApp extends React.Component<Props, RNTesterNavigationState> {
     });
   }
 
+  componentWillUnmount() {
+    this._mounted = false;
+  }
+
   _handleBack = () => {
     this._handleAction(RNTesterActions.Back());
   };