Fix missing import

Author: garyrussell

spring-integration-core/src/test/java/org/springframework/integration/dsl/flows/IntegrationFlowTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.integration.dsl.flows;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.junit.Assert.assertEquals;