Add missed import to IntegrationFlowTests

Signed-off-by: Artem Bilan <artem.bilan@broadcom.com>

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

@@ -95,6 +95,7 @@
 import org.springframework.messaging.support.ChannelInterceptor;
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.scheduling.TaskScheduler;
+import org.springframework.scheduling.concurrent.SimpleAsyncTaskScheduler;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
 import org.springframework.stereotype.Component;