Configure WebTestClient for @AutoConfigureMockMvc tests

[bclozel]

Spring Framework recently added WebTestClient support for testing Spring MVC applications using MockMvc (see spring-projects/spring-framework#19647).

Right now, Spring Boot offers two ways to test a web application: using TestRestTemplate on a running web server or with MockMvc (no actual server involved).

We should consider MockMvcTestClient and how we should update our testing infrastructure and documentation with this new option.

[Sam-Kruglov]

I can see there is already a MockMvcWebClientAutoConfiguration, why not create a similar MockMvcWebTestClientAutoConfiguration?
Here's how I'm doing it and it seems to work fine:

@BeforeEach
void setUp(@Autowired MockMvc mockMvc) {
    webTestClient = MockMvcWebTestClient.bindTo(mockMvc).build();
}

1 note: ParameterNamesModule missing from the default Jackson2JsonDecoder. Could use ObjectMapper#findAndRegisterModules