It's a minor bug in test case. `val testDir = null` will keep be `null` as it's immutable, so in finally block, nothing will be cleaned. Another `testDir` variable created in try block is only visible in try block.

## How was this patch tested?
Run existing test case and passed.

Author: mwws <wei.mao@intel.com>

Closes #12999 from mwws/SPARK_MINOR.