How to test a custom google signin change notifier class

I'm working on an app with Firebase, and I'm implementing tests on it. I'm new to it, and although I have seen several videos, examples and tutorials I can't find the proper way to do it.

  class GoogleSignInProvider extends ChangeNotifier {
  final googleSignIn = GoogleSignIn();

  GoogleSignInAccount? _user;
  GoogleSignInAccount get user => _user!;

  Future signInWithGoogle() async {
    try {
      // Trigger the authentication flow
      final GoogleSignInAccount? googleUser = await GoogleSignIn().signIn();
      if (googleUser == null) return;
      _user = googleUser;
      // Obtain the auth details from the request
      final GoogleSignInAuthentication? googleAuth =
          await googleUser.authentication;

      // Create a new credential
      final credential = GoogleAuthProvider.credential(
        accessToken: googleAuth?.accessToken,
        idToken: googleAuth?.idToken,
      );

      // Once signed in, return the UserCredential
      await FirebaseAuth.instance.signInWithCredential(credential);
    } on FirebaseAuthException catch (e) {
      return e.message;
    } catch (e) {
      rethrow;
    }
    notifyListeners();
  }

  Future logoutFromGoogle() async {
    await googleSignIn.disconnect();
    FirebaseAuth.instance.signOut();
  }
}

I already implemented mockito, but I can't (again) find the proper way to used it in my code. Any help?