Android: Test retrofit success,failure

I am new to Android testing. All I am trying now is new Espresso with Junit4. the thing I go till now is Espresso is for ui testing and with junit we can do logical testing. So I am trying Junit to test my retrofit code:

protected String signIn(String emailNumber, String password) {

    RestAdapter restAdapter = new RestAdapter.Builder().setEndpoint(Constants.API_URL).build();
    RetroInterface retroInterface = restAdapter.create(RetroInterface.class);

    retroInterface.signIn(emailNumber, password, new Callback<SignInPojo>() {
        @Override
        public void success(SignInPojo signInPojo, Response response) {
            if (signInPojo.getError()) {
                Snackbar.with(getApplicationContext()).text(signInPojo.getMessage())
                        .textColor(getResources().getColor(R.color.red_400)).show(SignInActivity.this);
                result = "successerror";

            } else {
                Log.d("Json", signInPojo.getName());
                result = "Successfully Signed In";
                editor = sharedPreferences.edit();
                editor.putBoolean(Constants.IS_LOGGED_IN, true);
                editor.apply();
                startActivity(new Intent(getApplicationContext(), LenderActivity.class));
                finish();
            }
        }

        @Override
        public void failure(RetrofitError error) {
            Log.d("RetroError", error.toString());
            Log.d("RetroUrl", error.getUrl());
            result = "failed";
        }
    });
    return result;
}

with these test Class: SignInActivityJunit.java

import android.support.test.InstrumentationRegistry;
import android.support.test.runner.AndroidJUnit4;
import android.test.ActivityInstrumentationTestCase2;
import android.test.suitebuilder.annotation.SmallTest;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

@RunWith(AndroidJUnit4.class)
@SmallTest
public class SignInActivityJunit extends ActivityInstrumentationTestCase2<SignInActivity>{
    private SignInActivity signInActivity;

    public SignInActivityJunit() {
        super(SignInActivity.class);
    }

    @Before
    public void setUp() throws Exception {
        super.setUp();
        injectInstrumentation(InstrumentationRegistry.getInstrumentation());
        signInActivity = getActivity();
    }

    @Test
    public void checkSignIn_Success() {
        String result = signInActivity.signIn("99929992", "aaaaaaaa");
        assertThat(result, is(equalTo("success")));
    }@Test
    public void checkSignIn_SuccessButError() {
        String result = signInActivity.signIn("99929992", "aaaaaaaa");
        assertThat(result, is(equalTo("successerror")));
    }@Test
    public void checkSignIn_Fail() {
        String result = signInActivity.signIn("99929992", "aaaaaaaa");
        assertThat(result, is(equalTo("success")));
    }

    @Override
    protected void tearDown() throws Exception {
        super.tearDown();
    }
}

Now these all cases failed because on debugging I saw that they are not waiting for the network to return call(As per my guess). They are skipping success and failure methods. So the question is.. how to make unit test wait till retrofit returns the request. Or is there any other efficient way to test these network connectivity. Thank you