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-test-nomockito/src/test/java/sample/testnomockito/SampleTestNoMockitoApplicationTests.java

@@ -16,12 +16,12 @@
 
 package sample.testnomockito;
 
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.context.ApplicationContext;
-import org.springframework.test.context.junit4.SpringRunner;
+import org.springframework.test.context.junit.jupiter.SpringExtension;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -32,8 +32,8 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Madhura Bhave
  */
-@RunWith(SpringRunner.class)
-public class SampleTestNoMockitoApplicationTests {
+@ExtendWith(SpringExtension.class)
+class SampleTestNoMockitoApplicationTests {
 
 	// gh-7065
 
@@ -41,7 +41,7 @@ public class SampleTestNoMockitoApplicationTests {
 	private ApplicationContext context;
 
 	@Test
-	public void contextLoads() throws Exception {
+	void contextLoads() throws Exception {
 		assertThat(this.context).isNotNull();
 	}