Add more documentation to the PartitionerTest

while working on https://issues.apache.org/jira/browse/SQOOP-1815, discovered a bug that failed the PartitionerTest.

1. the failure messages were hard to understand.

2. The actual problem was that there was a class cast exception or some other root cause. ..looks like this was a swallowed and the tests would fail since the HDFS output was not produced.