[SPARK-15508][STREAMING][TESTS] Fix flaky test: JavaKafkaStreamSuite.testKafkaStream

## What changes were proposed in this pull request?

`JavaKafkaStreamSuite.testKafkaStream` assumes when `sent.size == result.size`, the contents of `sent` and `result` should be same. However, that's not true. The content of `result` may not be the final content.

This PR modified the test to always retry the assertions even if the contents of `sent` and `result` are not same.

Here is the failure in Jenkins: http://spark-tests.appspot.com/tests/org.apache.spark.streaming.kafka.JavaKafkaStreamSuite/testKafkaStream

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #13281 from zsxwing/flaky-kafka-test.