GH-1289: Fix import in back-ported test

Author: garyrussell

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/core/RabbitTemplatePublisherCallbacksIntegrationTests2.java

@@ -17,8 +17,8 @@
 package org.springframework.amqp.rabbit.core;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
-import static org.junit.jupiter.api.Assertions.assertNotNull;
 
 import java.io.IOException;
 import java.util.concurrent.CountDownLatch;