[SPARK-5731][Streaming][Test] Fix incorrect test in DirectKafkaStreamSuite

The test was incorrect. Instead of counting the number of records, it counted the number of partitions of RDD generated by DStream. Which is not its intention. I will be testing this patch multiple times to understand its flakiness.

PS: This was caused by my refactoring in https://github.com/apache/spark/pull/4384/

koeninger check it out.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4597 from tdas/kafka-flaky-test and squashes the following commits:

d236235 [Tathagata Das] Unignored last test.
e9a1820 [Tathagata Das] fix test