I kept some of the sql import there to avoid changing too many lines.

Author: Reynold Xin <rxin@databricks.com>

Closes #6661 from rxin/remove-wildcard-import-sqlcontext and squashes the following commits:

c265347 [Reynold Xin] Fixed ListTablesSuite failure.
de9d491 [Reynold Xin] Fixed tests.
73b5365 [Reynold Xin] Mima.
8f6b642 [Reynold Xin] Fixed style violation.
443f6e8 [Reynold Xin] [SPARK-8113][SQL] Remove some wildcard import on TestSQLContext._