test: set FCM background handler in index.js per docs

this should have been in index.js so it could be registered as
a background handler prior to AppRegistry registration of the app

tests_react_native/example/app.tsx

@@ -89,16 +89,6 @@ async function onMessage(message: RemoteMessage): Promise<void> {
   // });
 }
 
-async function onBackgroundMessage(message: RemoteMessage): Promise<void> {
-  console.log('onBackgroundMessage New FCM Message', message);
-  // await Notifee.displayNotification({
-  //   title: 'onMessage',
-  //   body: `with message ${message.messageId}`,
-  //   android: { channelId: 'default', tag: 'hello1' },
-  // });
-}
-
-firebase.messaging().setBackgroundMessageHandler(onBackgroundMessage);
 function Root(): any {
   const [id, _] = React.useState<string | null>(null);
 

tests_react_native/index.js

@@ -1,8 +1,14 @@
 import { AppRegistry } from 'react-native';
+import '@react-native-firebase/messaging';
+import firebase from '@react-native-firebase/app';
 import notifee from '@notifee/react-native';
 
 import App from './example/app';
 
+firebase.messaging().setBackgroundMessageHandler(async message => {
+  console.log('onBackgroundMessage New FCM Message', message);
+});
+
 notifee.onBackgroundEvent(async event => {
   console.log('notifee.onBackgroundEvent triggered: ' + JSON.stringify(event));
 });