Fix KafkaDslTests for splitter race condition

It looks like `Stream.generate()` might producer items not in the expected order.
So, use `Stream.toList()` to be sure in the sequence size, and then `resequence()`
to be sure that items are emitted downstream in the proper sequence order.

Author: artembilan

spring-integration-kafka/src/test/java/org/springframework/integration/kafka/dsl/KafkaDslTests.java

@@ -349,7 +349,8 @@ public PollableChannel futuresChannel() {
 		public IntegrationFlow sendToKafkaFlow(
 				KafkaProducerMessageHandlerSpec<Integer, String, ?> kafkaMessageHandlerTopic2) {
 			return f -> f
-					.splitWith(s -> s.function(p -> Stream.generate(() -> p).limit(101).iterator()))
+					.splitWith(s -> s.function(p -> Stream.generate(() -> p).limit(101).toList()))
+					.resequence()
 					.enrichHeaders(h -> h.header(KafkaIntegrationHeaders.FUTURE_TOKEN, "foo"))
 					.publishSubscribeChannel(c -> c
 							.subscribe(sf -> sf.handle(