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-atmosphere/src/test/java/sample/atmosphere/SampleAtmosphereApplicationTests.java

@@ -22,8 +22,7 @@ import java.util.concurrent.atomic.AtomicReference;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.CommandLineRunner;
@@ -35,7 +34,6 @@ import org.springframework.boot.web.server.LocalServerPort;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
-import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.web.socket.TextMessage;
 import org.springframework.web.socket.WebSocketSession;
 import org.springframework.web.socket.client.WebSocketConnectionManager;
@@ -44,10 +42,9 @@ import org.springframework.web.socket.handler.TextWebSocketHandler;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-@RunWith(SpringRunner.class)
 @SpringBootTest(classes = SampleAtmosphereApplication.class,
 		webEnvironment = WebEnvironment.RANDOM_PORT)
-public class SampleAtmosphereApplicationTests {
+class SampleAtmosphereApplicationTests {
 
 	private static Log logger = LogFactory.getLog(SampleAtmosphereApplicationTests.class);
 
@@ -55,7 +52,7 @@ public class SampleAtmosphereApplicationTests {
 	private int port = 1234;
 
 	@Test
-	public void chatEndpoint() {
+	void chatEndpoint() {
 		ConfigurableApplicationContext context = new SpringApplicationBuilder(
 				ClientConfiguration.class, PropertyPlaceholderAutoConfiguration.class)
 						.properties("websocket.uri:ws://localhost:" + this.port