Configure WebTestClient for @AutoConfigureMockMvc tests

As of Spring Framework 5.3, `WebTestClient` can now be configured on top
of `MockMvc` for testing Spring MVC applications in a mock environment.

Prior to this commit, `WebTestClient` would be already configured for
WebFlux mock setups with `@AutoConfigureWebTestClient` or live servers
(for both MVC and WebFlux apps).

This commit enhances the `@AutoConfigureWebMvc` support so that a
`WebTestClient` instance is auto-configured if the spring-webflux
dependency is present on the classpath.

Closes gh-23067

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/testing/springbootapplications/withmockenvironment/MyMockMvcTests.java

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
 import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.web.reactive.server.WebTestClient;
 import org.springframework.test.web.servlet.MockMvc;
 
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
@@ -32,8 +33,20 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
 class MyMockMvcTests {
 
 	@Test
-	void exampleTest(@Autowired MockMvc mvc) throws Exception {
+	void testWithMockMvc(@Autowired MockMvc mvc) throws Exception {
 		mvc.perform(get("/")).andExpect(status().isOk()).andExpect(content().string("Hello World"));
 	}
 
+	// If Spring WebFlux is on the classpath, you can drive MVC tests with a WebTestClient
+	@Test
+	void testWithWebTestClient(@Autowired WebTestClient webClient) {
+		// @formatter:off
+		webClient
+				.get().uri("/")
+				.exchange()
+				.expectStatus().isOk()
+				.expectBody(String.class).isEqualTo("Hello World");
+		// @formatter:on
+	}
+
 }