PollerAdviceTests: synchronized list actions

https://build.spring.io/browse/INT-MASTERSPRING40-441

Even if we `stop()` a `SourcePollingChannelAdapter` that doesn't mean
that task-in-progress can't deliver its result to the source consumer.

* Wrap a list `add()` and iterator operations to avoid a
`ConcurrentModificationException`

Author: artembilan

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

@@ -257,7 +257,9 @@ public void testActiveIdleAdvice() throws Exception {
 		final LinkedList<Long> triggerPeriods = new LinkedList<>();
 		final DynamicPeriodicTrigger trigger = new DynamicPeriodicTrigger(10);
 		adapter.setSource(() -> {
-			triggerPeriods.add(trigger.getPeriod());
+			synchronized (triggerPeriods) {
+				triggerPeriods.add(trigger.getPeriod());
+			}
 			Message<Object> m = null;
 			if (latch.getCount() % 2 == 0) {
 				m = new GenericMessage<>("foo");
@@ -275,10 +277,9 @@ public void testActiveIdleAdvice() throws Exception {
 		adapter.start();
 		assertTrue(latch.await(10, TimeUnit.SECONDS));
 		adapter.stop();
-		while (triggerPeriods.size() > 5) {
-			triggerPeriods.removeLast();
+		synchronized (triggerPeriods) {
+			assertThat(triggerPeriods.subList(0, 5), contains(10L, 12L, 11L, 12L, 11L));
 		}
-		assertThat(triggerPeriods, contains(10L, 12L, 11L, 12L, 11L));
 	}
 
 	@Test
@@ -308,10 +309,7 @@ public void testCompoundTriggerAdvice() throws Exception {
 		assertTrue(latch.await(10, TimeUnit.SECONDS));
 		adapter.stop();
 		synchronized (overridePresent) {
-			while (overridePresent.size() > 5) {
-				overridePresent.removeLast();
-			}
-			assertThat(overridePresent, contains(null, override, null, override, null));
+			assertThat(overridePresent.subList(0, 5), contains(null, override, null, override, null));
 		}
 		verify(override, atLeast(2)).nextExecutionTime(any(TriggerContext.class));
 	}