MINOR: fix test after upstream change (#478)

Upstream fix: apache/kafka#14426

Author: mjsax

src/test/java/io/confluent/examples/streams/StreamToStreamJoinIntegrationTest.java

@@ -134,6 +134,7 @@ public void shouldJoinTwoStreams() {
       impressionInput.pipeKeyValueList(inputAdImpressions);
       clickInput.pipeKeyValueList(inputAdClicks);
       // Flush result
+      topologyTestDriver.advanceWallClockTime(Duration.ofSeconds(1L));
       impressionInput.pipeInput("dummy", "dummy", currentMs + windowSize.toMillis() * 3);
 
       //