KAFKA-8091; Use commitSync to check connection failure in listener update test (#6450)

The use of consumer.poll() made the test flaky since in some cases, it doesn't wait for coordinator connection.

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>

core/src/test/scala/integration/kafka/server/DynamicBrokerReconfigurationTest.scala

@@ -1315,7 +1315,7 @@ class DynamicBrokerReconfigurationTest extends ZooKeeperTestHarness with SaslSet
     executors += executor
     val future = executor.submit(new Runnable() {
       def run() {
-        assertEquals(0, consumer.poll(100).count)
+        consumer.commitSync()
       }
     })
     verifyTimeout(future)