发布我自己的答案以防其他人受益！
底线是你不再需要初始化应用程序来进行测试
Firebase.initializeApp(); //not required
相反，您必须确保在测试期间在应用程序中调用firebase时，它提供了一个合适的mock。如果提供了合适的mock，则可以避免错误消息：
[core/no-app] No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()
如何提供模拟的示例如下所示

import 'package:fake_cloud_firestore/fake_cloud_firestore.dart';

    final FakeFirebaseFirestore fakeFirebaseFirestoreManufacturersCollection = FakeFirebaseFirestore();

    initialiseFirestoreManufacturersCollectionFake() async {
  List<Manufacturer> testManufacturerList = [];

  testManufacturerList.add(Manufacturer(
      keyword: "none",
      id: "testManf1",
      countryOfOrigin: "United Kingdom",
      manufacturingLocations: [],
      distribution: [],
  ));

  testManufacturerList.forEach((manufacturer) async {
    await fakeFirebaseFirestoreManufacturersCollection
        .collection(FirestoreManufacturerUtils.manufacturersCollection)
        .doc(manufacturer.id)
        .set(manufacturer.toJson());
  });
}

main(){

  TestWidgetsFlutterBinding.ensureInitialized();
  
  

  setUpAll(() async {
    //await Firebase.initializeApp(); //no need for this anymore!
  });

  Widget testAppMultiTab({Widget? child}){

    ///before anything initialise the Firestore singleton with a Fake
    initialiseFirestoreManufacturersCollectionFake();
    var fakeFirebaseFirestoreManufacturersCollection = FirestoreFakerUtil.fakeFirebaseFirestoreManufacturersCollection;
    manufacturerInfo = ManufacturerInformation(firestore: fakeFirebaseFirestoreManufacturersCollection);
    
    return MaterialApp(
      home: Scaffold(
          body: ScopedModel<ManufacturerInformation>(model: manufacturerInfo,child: MaterialApp(
                        title: 'Manufacturing app',
                        home: HomeScreen(),
                      ),
                    ),
                  ),
                ),
              ))
      ),
    );
  }

  group("some tests which involve firestore calls", (){
  //...
  })
}

”
希望这能帮助到外面的人！！

是的，在firebase核心平台接口API中有一些突破性的变化。
您需要添加以下内容。会成功的

import 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart';
import 'package:flutter/services.dart';
import 'package:flutter_test/flutter_test.dart';

void setupFirebaseAuthMocks([Callback? customHandlers]) {
  TestWidgetsFlutterBinding.ensureInitialized();

  setupFirebaseCoreMocks();
}

您可以删除以下方法，因为channel已从最新依赖项中删除。

MethodChannelFirebase.channel.setMockMethodCallHandler((call) async { 

});