[SPARK-6765] Enable scalastyle on test code.

Turn scalastyle on for all test code. Most of the violations have been resolved in my previous pull requests:

Core: https://github.com/apache/spark/pull/5484
SQL: https://github.com/apache/spark/pull/5412
MLlib: https://github.com/apache/spark/pull/5411
GraphX: https://github.com/apache/spark/pull/5410
Streaming: https://github.com/apache/spark/pull/5409

Author: Reynold Xin <rxin@databricks.com>

Closes #5486 from rxin/test-style-enable and squashes the following commits:

01683de [Reynold Xin] Fixed new code.
a4ab46e [Reynold Xin] Fixed tests.
20adbc8 [Reynold Xin] Missed one violation.
5e36521 [Reynold Xin] [SPARK-6765] Enable scalastyle on test code.