Validate our own tests work with JUnit5 and the vintage engine

Closes gh-14737

Co-authored-by: Stephane Nicoll <snicoll@pivotal.io>

spring-boot-samples/spring-boot-sample-webflux/src/test/java/sample/webflux/SampleWebFluxApplicationTests.java

@@ -16,15 +16,13 @@
 
 package sample.webflux;
 
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
 import org.springframework.http.MediaType;
-import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.test.web.reactive.server.WebTestClient;
 
 /**
@@ -32,21 +30,20 @@ import org.springframework.test.web.reactive.server.WebTestClient;
  *
  * @author Brian Clozel
  */
-@RunWith(SpringRunner.class)
 @SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
-public class SampleWebFluxApplicationTests {
+class SampleWebFluxApplicationTests {
 
 	@Autowired
 	private WebTestClient webClient;
 
 	@Test
-	public void testWelcome() {
+	void testWelcome() {
 		this.webClient.get().uri("/").accept(MediaType.TEXT_PLAIN).exchange()
 				.expectBody(String.class).isEqualTo("Hello World");
 	}
 
 	@Test
-	public void testEcho() {
+	void testEcho() {
 		this.webClient.post().uri("/echo").contentType(MediaType.TEXT_PLAIN)
 				.accept(MediaType.TEXT_PLAIN)
 				.body(Mono.just("Hello WebFlux!"), String.class).exchange()
@@ -54,7 +51,7 @@ public class SampleWebFluxApplicationTests {
 	}
 
 	@Test
-	public void testActuatorStatus() {
+	void testActuatorStatus() {
 		this.webClient.get().uri("/actuator/health").accept(MediaType.APPLICATION_JSON)
 				.exchange().expectStatus().isOk().expectBody()
 				.json("{\"status\":\"UP\"}");