@Test
public void testStopAsyncShouldStopWebAppServer() {

    //GIVEN
    ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);
    //WHEN
    underTest.stopAsync();
    //THEN
    verify(executorService).execute(captor.capture());
    captor.getValue().run();
    verify(webAppServer).stop();
}