Add missing Kotlin example for MockMvc and RestTestClient

Closes gh-47371

documentation/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/testing/springbootapplications/withmockenvironment/MyMockMvcTests.kt

@@ -24,6 +24,8 @@ import org.springframework.boot.test.context.SpringBootTest
 import org.springframework.test.web.reactive.server.WebTestClient
 import org.springframework.test.web.reactive.server.expectBody
 import org.springframework.test.web.servlet.assertj.MockMvcTester
+import org.springframework.test.web.servlet.client.RestTestClient
+import org.springframework.test.web.servlet.client.expectBody
 
 @SpringBootTest
 @AutoConfigureMockMvc
@@ -35,6 +37,15 @@ class MyMockMvcTests {
 				.hasBodyTextEqualTo("Hello World")
 	}
 
+	@Test
+	fun testWithRestTestClient(@Autowired webClient: RestTestClient) {
+		webClient
+			.get().uri("/")
+			.exchange()
+			.expectStatus().isOk
+			.expectBody<String>().isEqualTo("Hello World")
+	}
+
 	// If Spring WebFlux is on the classpath, you can drive MVC tests with a WebTestClient
 
 	@Test