Fix Race in ReactiveStreamsTests

Messages sent/published before the async task has fully set up, thus it does
not receive the messages.

Author: garyrussell

spring-integration-core/src/test/java/org/springframework/integration/dsl/reactivestreams/ReactiveStreamsTests.java

@@ -115,6 +115,8 @@ public void testPollableReactiveFlow() throws InterruptedException, TimeoutExcep
 				.take(6)
 				.subscribe();
 
+		final CountDownLatch asyncSetupLatch = new CountDownLatch(1);
+
 		Future<List<Integer>> future =
 				Executors.newSingleThreadExecutor().submit(() ->
 						Flux.just("11,12,13")
@@ -126,7 +128,9 @@ public void testPollableReactiveFlow() throws InterruptedException, TimeoutExcep
 								.map(Message::getPayload)
 								.take(7)
 								.collectList()
-								.block(Duration.ofSeconds(10)));
+								.block(getDuration(asyncSetupLatch)));
+
+		assertTrue(asyncSetupLatch.await(10, TimeUnit.SECONDS));
 
 		this.inputChannel.send(new GenericMessage<>("6,7,8,9,10"));
 
@@ -136,6 +140,11 @@ public void testPollableReactiveFlow() throws InterruptedException, TimeoutExcep
 		assertEquals(7, integers.size());
 	}
 
+	private Duration getDuration(CountDownLatch asyncSetupLatch) {
+		asyncSetupLatch.countDown();
+		return Duration.ofSeconds(10);
+	}
+
 	@Configuration
 	@EnableIntegration
 	public static class ContextConfiguration {