Fix import in KafkaProducerMessageHandlerTests

Author: artembilan

spring-integration-kafka/src/test/java/org/springframework/integration/kafka/outbound/KafkaProducerMessageHandlerTests.java

@@ -23,6 +23,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;