Stubbing deep chaining calls about fake_cloud_firestore

[dfdgsdfg]

Add snapshots() method to mock class seems fine.

  @override
  Stream<DocumentSnapshot> snapshots({bool includeMetadataChanges = false}) {
    return Stream.value(MockDocumentSnapshot(_documentId, root));
  }

Then test like below.

    test('return right item', () async {
      final firestore = MockFirestoreInstance();

      await firestore.collection('A').document('1').setData(preparedData1);
      await firestore.collection('A').document('2').setData(preparedData2);

      final api = SomeApi(firestore);
      api.getStreamA("1").listen(expectAsync1((res) {
        expect(res.documentID, equals("1"));
      }));
      //   Stream<DocumentSnapshot> getStreamA(String id) => _db.collection('A').document(id).snapshots();