[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.

python/run-tests.py

@@ -56,7 +56,8 @@ LOGGER = logging.getLogger()
 
 def run_individual_python_test(test_name, pyspark_python):
     env = dict(os.environ)
-    env.update({'SPARK_TESTING': '1', 'PYSPARK_PYTHON': which(pyspark_python)})
+    env.update({'SPARK_TESTING': '1', 'PYSPARK_PYTHON': which(pyspark_python),
+                'PYSPARK_DRIVER_PYTHON': which(pyspark_python)})
     LOGGER.debug("Starting test(%s): %s", pyspark_python, test_name)
     start_time = time.time()
     try: