MINOR: Fix transient test failure in DynamicConnectionQuotaTest (#5544)

Reviewers: Jason Gustafson <jason@confluent.io>

core/src/test/scala/integration/kafka/network/DynamicConnectionQuotaTest.scala

@@ -76,6 +76,7 @@ class DynamicConnectionQuotaTest extends BaseRequestTest {
     val (tp, partitionResponse) = produceResponse.responses.asScala.head
     assertEquals(Errors.NONE, partitionResponse.error)
 
+    TestUtils.waitUntilTrue(() => connectionCount == (maxConnectionsPerIP - 1), "produce request connection is not closed")
     conns = conns :+ connect(socketServer)
     // now try one more (should fail)
     intercept[IOException](sendProduceRequest())
@@ -100,12 +101,14 @@ class DynamicConnectionQuotaTest extends BaseRequestTest {
     val (tp1, partitionResponse1) = produceResponse.responses.asScala.head
     assertEquals(Errors.NONE, partitionResponse1.error)
 
+    TestUtils.waitUntilTrue(() => connectionCount == (maxConnectionsPerIPOverride - 1), "produce request connection is not closed")
     conns = conns :+ connect(socketServer)
     // now try one more (should fail)
     intercept[IOException](sendProduceRequest())
 
     //close one connection
     conns.head.close()
+    TestUtils.waitUntilTrue(() => connectionCount == (maxConnectionsPerIPOverride - 1), "connection is not closed")
     // send should succeed
     sendProduceRequest()
   }