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-servlet/src/test/java/sample/servlet/SampleServletApplicationTests.java

@@ -18,8 +18,7 @@ package sample.servlet;
 
 import java.util.Collections;
 
-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;
@@ -31,7 +30,6 @@ import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
-import org.springframework.test.context.junit4.SpringRunner;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -40,15 +38,14 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Dave Syer
  */
-@RunWith(SpringRunner.class)
 @SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
-public class SampleServletApplicationTests {
+class SampleServletApplicationTests {
 
 	@Autowired
 	private TestRestTemplate restTemplate;
 
 	@Test
-	public void testHomeIsSecure() {
+	void testHomeIsSecure() {
 		HttpHeaders headers = new HttpHeaders();
 		headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
 		ResponseEntity<String> entity = this.restTemplate.exchange("/", HttpMethod.GET,
@@ -57,7 +54,7 @@ public class SampleServletApplicationTests {
 	}
 
 	@Test
-	public void testHome() {
+	void testHome() {
 		ResponseEntity<String> entity = this.restTemplate
 				.withBasicAuth("user", getPassword()).getForEntity("/", String.class);
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);