How can I run code, that will call AsyncTask extended class, under UI Thread in Unit testing class in Android

First, let me give you the scenario:

interface TaskDone{

void OnTaskDone(Object data)

}

public class DownloadImage extends AsyncTask{

private TaskDone callback;
public DownloadImage(TaskDone callback){this.callback=callback}

@Override protected Bitmap doInBackground(String ...params){

Bitmap image= null;

// contact to server and download image

return image;

}

@Override protected void onPostExecute(Bitmap result) {

    super.onPostExecute(result);
    if (result != null) {
        callback.onTaskDone(result);
    }
}

}

Now I want to do Unit test that the image is being downloaded. As it is mandatory that the DownloadImage class need to be called from UI thread and I don't have any Activity class in my source packages. How can I write my unit testing? I have done something like the following but it seems to me that runOnUIThread is not running.

class DownloadImageTest {

@Before public void setUp() throws Exception {

    countDownLatch = new CountDownLatch(1);

}

public void test1_TaskDownload_should_download_a_lanscape_banner() throws Throwable {

Context context = Mockito.mock(Context.class);

Context contextSpy = PowerMockito.spy(context);

Intent intent = new Intent(contextSpy, Activity.class);

contextSpy.startActivity(intent);

activity.startActivity(intent);

activity.runOnUiThread(new Runnable() {

        @Override
        public void run() {
            DownloadImage downloadImage = new DownloadImage(new TaskDone() {
                @Override
public void onTaskDone(Object data) {

rawImageLandscape = (Bitmap) data;
countDownLatch.countDown();
}
});

downloadImage.execute(URL);
}
});

try {

  countDownLatch.await(5, TimeUnit.SECONDS);

} catch (InterruptedException e) {

   e.printStackTrace();
}

}

}

Any help would be greatly appreciated. Thanks for reading the question too.enter code here