Fix unit tests

Author: collinjackson

packages/firebase_in_app_messaging/test/firebase_in_app_messaging_test.dart

@@ -7,6 +7,8 @@ import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
   group('$FirebaseInAppMessaging', () {
     final List<MethodCall> log = <MethodCall>[];