Unit Testing Android app that relies on RxJava / RxAndroid

I have a test:

    @Test
    public void valid_login_transitions_to_main_app() throws Exception {
        when(testReferenceManager.getUserServiceMock().checkUsernameAvailability(anyString())).thenReturn(Observable.just(Arrays.asList(new User("test@email.com", "password"))));
        when(testReferenceManager.getUserServiceMock().checkAuthStatus(anyString())).thenReturn(Observable.just(Arrays.asList(new User("test@email.com", "password"))));

        EditText emailText = (EditText)activity.findViewById(R.id.text_email);
        EditText passwordText = (EditText)activity.findViewById(R.id.text_password);
        Button signInButton = (Button)activity.findViewById(R.id.sign_in_button);

        emailText.setText("test@email.com");
        passwordText.setText("password");


        assertThat(signInButton.getText()).isEqualTo(App.R.getString(R.string.button_login));
}

The key thing here is that if the API reports a user exists (which it does from the mocks above) the sign in button text should be the same as the value of the string resource named R.string.button_login.

The setup for changing the button state is done in my Activity like this:

        ReactiveEditText.textObservableForTextView(emailText)
            .startWith(emailText.getText().toString())
            .subscribe(new EndlessObserver<String>() {
                @Override
                public void onNext(String username) {
                    setSignInEnabledState();
                    webServices.usernameAvailable(username)
                            .observeOn(Schedulers.immediate())
                            .subscribe(new EndlessObserver<Boolean>() {
                                @Override
                                public void onNext(Boolean available) {           
                                    usernameIsAvailable = available;
                                    if (available) {                                        
                                            signInButton.setText(getString(R.string.button_signup));                                            
                                        } else {
                                            signInButton.setText(getString(R.string.button_login));                                           
                                        }
                                    }
                                });
                    }
                });

ReactiveEditText.textObservableForTextView simply wraps the textChangedListener interface in a reactive fashion:

    public static Observable<String> textObservableForTextView(final TextView tv) {
    return Observable.create(new Observable.OnSubscribe<String>() {
        @Override
        public void call(final Subscriber<? super String> subscriber) {
            tv.addTextChangedListener(new TextWatcher() {
                @Override public void beforeTextChanged(CharSequence s, int start, int count, int after) {}
                @Override public void afterTextChanged(Editable s) { }

                @Override
                public void onTextChanged(CharSequence s, int start, int before, int count) {
                    subscriber.onNext(s.toString());
                }
            });
        }
    });
}

Notice that I am using the 'immediate' scheduler to listen for changes to the EditText. My confusion is this: the unit test always fails, and it seems to me (when I jump into the debugger) that the assertThat statement in the test is firing before the observer ever sees a change in the EditText. I do see in the debugger that the Observer does eventually fire and the text is properly set on the signInButton. I thought using the immediate scheduler would make everything happen as I expect directly after setText is called.