Avoid Race in FtpStreamingMessageSourceTests

See #2776

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/inbound/FtpStreamingMessageSourceTests.java

@@ -174,7 +174,7 @@ public QueueChannel data() {
 		public PollerMetadata defaultPoller() {
 			PollerMetadata pollerMetadata = new PollerMetadata();
 			pollerMetadata.setTrigger(new PeriodicTrigger(500));
-			pollerMetadata.setMaxMessagesPerPoll(2000);
+			pollerMetadata.setMaxMessagesPerPoll(1);
 			return pollerMetadata;
 		}