Add 1 sec delay to the ReactiveStreamsTests.reactiveFlow poller

Looks like there is some race condition in starting endpoints in the flow,
and poller was able to emit message before downstream channel is subscribed

Author: artembilan

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

@@ -306,7 +306,9 @@ public static class ContextConfiguration {
 		public Publisher<Message<String>> reactiveFlow() {
 			return IntegrationFlow
 					.from(() -> new GenericMessage<>("a,b,c,d,e,f"),
-							e -> e.poller(p -> p.trigger(ctx -> this.invoked.getAndSet(true) ? null : Instant.now()))
+							e -> e.poller(p ->
+											p.trigger(ctx ->
+													this.invoked.getAndSet(true) ? null : Instant.now().plusMillis(1000)))
 									.sendTimeout(256)
 									.id("reactiveStreamsMessageSource"))
 					.split(String.class, p -> p.split(","))