@Test
public void testHelloWorldController() throws Exception {

   MvcResult mvcResult = mockMvc.perform(MockMvcRequestBuilders.get("/"))
           .andExpect(request().asyncStarted())
           .andDo(MockMvcResultHandlers.print())
           .andReturn();

   mockMvc.perform(asyncDispatch(mvcResult))
           .andExpect(status().isOk())
           .andExpect(content().contentTypeCompatibleWith(MediaType.TEXT_PLAIN))
           .andExpect(content().string("Hello World, from Spring Boot 2!"));
}