[SPARK-12361][PYSPARK][TESTS] Should set PYSPARK_DRIVER_PYTHON before Python tests

Although this patch still doesn't solve the issue why the return code is 0 (see JIRA description), it resolves the issue of python version mismatch.

Author: Jeff Zhang <zjffdu@apache.org>

Closes #10322 from zjffdu/SPARK-12361.