Add JUnit 5 checkstyle rules

Add a rule to enforce JUnit 5 usage and conventions.

Closes gh-17093

spring-boot-tests/spring-boot-integration-tests/spring-boot-hibernate52-tests/src/test/java/org/springframework/boot/tests/hibernate52/Hibernate52ApplicationTests.java

@@ -21,11 +21,10 @@ import org.junit.jupiter.api.Test;
 import org.springframework.boot.test.context.SpringBootTest;
 
 @SpringBootTest
-public class Hibernate52ApplicationTests {
+class Hibernate52ApplicationTests {
 
 	@Test
-	public void contextLoads() {
-
+	void contextLoads() {
 	}
 
 }