We set `sqlContext = null` in `afterAll`. However, this doesn't change `SQLContext.activeContext`  and then `SQLContext.getOrCreate` might use the `SparkContext` from previous test suite and hence causes the error. This PR calls `clearActive` in `beforeAll` and `afterAll` to avoid using an old context from other test suites.

cc: yhuai

Author: Xiangrui Meng <meng@databricks.com>

Closes #9677 from mengxr/SPARK-11672.2.