Update MSKFirehoseEventHandlerTest.java

Added license text.

Author: ShashankAWS

samples/msk-firehose-event-handler/src/test/java/example/MSKFirehoseEventHandlerTest.java

@@ -1,3 +1,8 @@
+/*
+Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+SPDX-License-Identifier: Apache-2.0
+*/
+
 package example;
 
 import com.amazonaws.services.lambda.runtime.Context;