@Test
public void execute_withEmptyUrlList_shouldReturnEmptyMap() throws Exception {

    subject = new ImageDownloadTaskManager(
            new ArrayList<String>(),
            mMockImageTaskManagerListener,
            testMaxWidth
    );
    subject.execute();
    semaphore.acquire();
    assertThat(networkImages.isEmpty()).isTrue();
    verify(mMockImageTaskManagerListener).onSuccess(anyMap());
    verify(mMockImageTaskManagerListener, never()).onFail();
}