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-war/src/test/java/sample/war/WarApplicationResourceTests.java

@@ -16,18 +16,15 @@
 
 package sample.war;
 
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.test.context.SpringBootTest;
-import org.springframework.test.context.junit4.SpringRunner;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-@RunWith(SpringRunner.class)
 @SpringBootTest
-public class WarApplicationResourceTests {
+class WarApplicationResourceTests {
 
 	// gh-6371
 
@@ -35,7 +32,7 @@ public class WarApplicationResourceTests {
 	private String demoStringValue;
 
 	@Test
-	public void contextLoads() {
+	void contextLoads() {
 		assertThat(this.demoStringValue).isEqualTo("demo");
 	}