SPARK-2283: 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.