Fix RotatingServersTests concurrency

https://build.spring.io/browse/INT-FATS5IC-685/

Even if we stop `SourcePollingChannelAdapter`, the task on the fly is
still going to be executed.
This way we may have extra calls to our logic.

* Use `CopyOnWriteArrayList` for the `sessionSources` to avoid
`ConcurrentModificationException` during this collection read in the
tests

**Cherry-pick to 5.0.x**

(cherry picked from commit 76c6700)

Author: artembilan

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

@@ -22,6 +22,7 @@
 import java.io.File;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Collectors;
@@ -168,7 +169,7 @@ public static class StandardConfig {
 
 		private final CountDownLatch latch = new CountDownLatch(17);
 
-		List<Integer> sessionSources = new ArrayList<>();
+		final List<Integer> sessionSources = new CopyOnWriteArrayList<>();
 
 		@Bean
 		public SessionFactory<FTPFile> factory1() {