Как протестировать статические методы в Android JUNIT тестами

public class FirebaseServer {


    public static final String TAG = FirebaseServer.class.getSimpleName();

    private static FirebaseAuth mAuth = FirebaseAuth.getInstance();

    public static void registerNewUser(String email, String password, FirebaseRegisterListener listener){
        mAuth.createUserWithEmailAndPassword(email,password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if(task.isSuccessful()){
                    Log.e(TAG,"register new user success");
                    listener.onRegisterSuccess(task.getResult());
                }else {
                    Log.e(TAG,"register new user failure");
                    listener.onRegisterFailure(task.getException().toString());
                }
            }
        });
    }

    public static void signIn(String email, String password, FirebaseSignInListener listener){
        mAuth.signInWithEmailAndPassword(email,password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if(task.isSuccessful()){
                    Log.e(TAG,"SignIn in Firebase is successful");
                    listener.signInSuccess(task.getResult());
                }else {
                    Log.e(TAG,"SignIn in Firebase is failure");
                    listener.signInFailure(task.getException().toString());
                }
            }
        });
    }
}