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-web-jsp/src/test/java/sample/jsp/SampleWebJspApplicationTests.java

@@ -19,8 +19,7 @@ package sample.jsp;
 import java.net.URI;
 import java.util.Arrays;
 
-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.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
-import org.springframework.test.context.junit4.SpringRunner;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -41,22 +39,21 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Phillip Webb
  */
-@RunWith(SpringRunner.class)
 @SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
-public class SampleWebJspApplicationTests {
+class SampleWebJspApplicationTests {
 
 	@Autowired
 	private TestRestTemplate restTemplate;
 
 	@Test
-	public void testJspWithEl() {
+	void testJspWithEl() {
 		ResponseEntity<String> entity = this.restTemplate.getForEntity("/", String.class);
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
 		assertThat(entity.getBody()).contains("/resources/text.txt");
 	}
 
 	@Test
-	public void customErrorPage() throws Exception {
+	void customErrorPage() throws Exception {
 		HttpHeaders headers = new HttpHeaders();
 		headers.setAccept(Arrays.asList(MediaType.TEXT_HTML));
 		RequestEntity<Void> request = new RequestEntity<>(headers, HttpMethod.GET,