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-static/src/test/java/sample/web/staticcontent/SampleWebStaticApplicationTests.java

@@ -16,8 +16,7 @@
 
 package sample.web.staticcontent;
 
-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;
@@ -26,7 +25,6 @@ import org.springframework.boot.test.web.client.TestRestTemplate;
 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;
 
@@ -35,22 +33,21 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Dave Syer
  */
-@RunWith(SpringRunner.class)
 @SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
-public class SampleWebStaticApplicationTests {
+class SampleWebStaticApplicationTests {
 
 	@Autowired
 	private TestRestTemplate restTemplate;
 
 	@Test
-	public void testHome() {
+	void testHome() {
 		ResponseEntity<String> entity = this.restTemplate.getForEntity("/", String.class);
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
 		assertThat(entity.getBody()).contains("<title>Static");
 	}
 
 	@Test
-	public void testCss() {
+	void testCss() {
 		ResponseEntity<String> entity = this.restTemplate.getForEntity(
 				"/webjars/bootstrap/3.0.3/css/bootstrap.min.css", String.class);
 		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);