Using ActivityInstrumentationTestCase2 to select a menu item?

I am trying to write test cases for my main activity. All my main activity shows right now is a list view, and I was able to write tests to start the activity and make sure the list view was created by doing this:

public class MainActivityTest extends ActivityInstrumentationTestCase2<MainActivity>{
    private MainActivity mainActivity;
    private ListView mListView;

    public MainActivityTest(){
        super("MainActivityTest", MainActivity.class);
    }

    @Override
    protected void setUp() throws Exception {
        super.setUp();
        mainActivity = this.getActivity();
        mListView = (ListView) mainActivity.findViewById(R.id.prescriptionListView);
    }

    public void testPreconditions(){
        assertNotNull(mListView);
    }
}

What I would like to do next is open the options menu and select an item. When that item is selected, it should create an intent and start a new activity. However, I can't figure out how to reference the menu. I have tried a few things:

public void testOptionItem(){
    // Not sure how to use this
    mainActivity.getMenuInflater();

    // Not sure if this actually opens the menu
    // app closes to quickly, but even if it does I don't know how to select item
    mainActivity.openOptionsMenu();

    // I get a null reference exception on the menu item.
    mainActivity.onOptionsItemSelected((MenuItem) mainActivity.findViewById(R.id.action_addPrescription));
}

How can I obtain a reference to the menu and select an item?