Synchronize list of events for FTP server tests

Author: artembilan

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/outbound/FtpServerOutboundTests.java

@@ -34,6 +34,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Calendar;
+import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.BlockingQueue;
@@ -800,7 +801,7 @@ public Object doInSession(Session<FTPFile> session, Message<?> requestMessage) t
 	@Component
 	public static class Config {
 
-		final List<ApacheMinaFtpEvent> events = new ArrayList<>();
+		final List<ApacheMinaFtpEvent> events = Collections.synchronizedList(new ArrayList<>());
 
 		private volatile String targetLocalDirectoryName;