[SPARK-2328] [SQL] Add execution of `SHOW TABLES` before `TestHive.reset()`.

`PruningSuite` is executed first of Hive tests unfortunately, `TestHive.reset()` breaks the test environment.
To prevent this, we must run a query before calling reset the first time.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1268 from ueshin/issues/SPARK-2328 and squashes the following commits:

043ceac [Takuya UESHIN] Add execution of `SHOW TABLES` before `TestHive.reset()`.