Wrap LinkedList manipulations into synchronized

https://build.spring.io/browse/INT-MASTER-1017

Looks like non-`volatile` `size` property in the `LinkedList` is not
updated properly for the `contains()` assertion, therefore we end up
with the `ConcurrentModificationException`

* Move `assertThat()` into the `synchronized (overridePresent)` block
to enforce CPU cache flushing and, therefore, have an actual value
for the `LinkedList.size` during iteration

spring-integration-core/src/test/java/org/springframework/integration/endpoint/PollerAdviceTests.java

@@ -311,8 +311,8 @@ public class PollerAdviceTests {
 			while (overridePresent.size() > 5) {
 				overridePresent.removeLast();
 			}
+			assertThat(overridePresent, contains(null, override, null, override, null));
 		}
-		assertThat(overridePresent, contains(null, override, null, override, null));
 		verify(override, atLeast(2)).nextExecutionTime(any(TriggerContext.class));
 	}