[SPARK-3399][PySpark] Test for PySpark should ignore HADOOP_CONF_DIR and YARN_CONF_DIR

Thanks for your comment.
As I mentioned in JIRA, YARN_CONF_DIR and HADOOP_CONF_DIR is loaded in pyspark script and some tests like rdd.py are kicked by pyspark in python/run-tests so it's not make sense that putting "unset" on python/run-tests.