[SPARK-2283][SQL] Reset test environment before running PruningSuite

JIRA issue: [SPARK-2283](https://issues.apache.org/jira/browse/SPARK-2283)

If `PruningSuite` is run right after `HiveCompatibilitySuite`, the first test case fails because `srcpart` table is cached in-memory by `HiveCompatibilitySuite`, but column pruning is not implemented for `InMemoryColumnarTableScan` operator yet.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1221 from liancheng/spark-2283 and squashes the following commits:

dc0b663 [Cheng Lian] SPARK-2283: reset test environment before running PruningSuite

(cherry picked from commit 7f196b009d26d4aed403b3c694f8b603601718e3)
Signed-off-by: Michael Armbrust <michael@databricks.com>