[SPARK-18588][TESTS] Fix flaky test: KafkaSourceStressForDontFailOnDataLossSuite

## What changes were proposed in this pull request?

Fixed the following failures:

```
org.scalatest.exceptions.TestFailedDueToTimeoutException: The code passed to eventually never returned normally. Attempted 3745 times over 1.0000790851666665 minutes. Last failure message: assertion failed: failOnDataLoss-0 not deleted after timeout.
```

```
sbt.ForkMain$ForkError: org.apache.spark.sql.streaming.StreamingQueryException: Query query-66 terminated with exception: null
	at org.apache.spark.sql.execution.streaming.StreamExecution.org$apache$spark$sql$execution$streaming$StreamExecution$$runBatches(StreamExecution.scala:252)
	at org.apache.spark.sql.execution.streaming.StreamExecution$$anon$1.run(StreamExecution.scala:146)
Caused by: sbt.ForkMain$ForkError: java.lang.NullPointerException: null
	at java.util.ArrayList.addAll(ArrayList.java:577)
	at org.apache.kafka.clients.Metadata.getClusterForCurrentTopics(Metadata.java:257)
	at org.apache.kafka.clients.Metadata.update(Metadata.java:177)
	at org.apache.kafka.clients.NetworkClient$DefaultMetadataUpdater.handleResponse(NetworkClient.java:605)
	at org.apache.kafka.clients.NetworkClient$DefaultMetadataUpdater.maybeHandleCompletedReceive(NetworkClient.java:582)
	at org.apache.kafka.clients.NetworkClient.handleCompletedReceives(NetworkClient.java:450)
	at org.apache.kafka.clients.NetworkClient.poll(NetworkClient.java:269)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.clientPoll(ConsumerNetworkClient.java:360)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:224)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.poll(ConsumerNetworkClient.java:192)
	at org.apache.kafka.clients.consumer.internals.ConsumerNetworkClient.awaitPendingRequests(ConsumerNetworkClient.java:260)
	at org.apache.kafka.clients.consumer.internals.AbstractCoordinator.ensureActiveGroup(AbstractCoordinator.java:222)
	at org.apache.kafka.clients.consumer.internals.ConsumerCoordinator.ensurePartitionAssignment(ConsumerCoordinator.java:366)
	at org.apache.kafka.clients.consumer.KafkaConsumer.pollOnce(KafkaConsumer.java:978)
	at org.apache.kafka.clients.consumer.KafkaConsumer.poll(KafkaConsumer.java:938)
	at
...
```

## How was this patch tested?

Tested in #16048 by running many times.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16109 from zsxwing/fix-kafka-flaky-test.

(cherry picked from commit edc87e18)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>