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-undertow/src/test/java/sample/undertow/SampleUndertowApplicationTests.java

@@ -20,8 +20,7 @@ import java.io.ByteArrayInputStream;
 import java.nio.charset.StandardCharsets;
 import java.util.zip.GZIPInputStream;
 
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
@@ -32,7 +31,6 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
-import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.util.StreamUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -43,25 +41,24 @@ import static org.assertj.core.api.Assertions.assertThat;
  * @author Ivan Sopov
  * @author Andy Wilkinson
  */
-@RunWith(SpringRunner.class)
 @SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
-public class SampleUndertowApplicationTests {
+class SampleUndertowApplicationTests {
 
 	@Autowired
 	private TestRestTemplate restTemplate;
 
 	@Test
-	public void testHome() {
+	void testHome() {
 		assertOkResponse("/", "Hello World");
 	}
 
 	@Test
-	public void testAsync() {
+	void testAsync() {
 		assertOkResponse("/async", "async: Hello World");
 	}
 
 	@Test
-	public void testCompression() throws Exception {
+	void testCompression() throws Exception {
 		HttpHeaders requestHeaders = new HttpHeaders();
 		requestHeaders.set("Accept-Encoding", "gzip");
 		HttpEntity<?> requestEntity = new HttpEntity<>(requestHeaders);