[SPARK-15975] Fix improper Popen retcode code handling in dev/run-tests

In the `dev/run-tests.py` script we check a `Popen.retcode` for success using `retcode > 0`, but this is subtlety wrong because Popen's return code will be negative if the child process was terminated by a signal: https://docs.python.org/2/library/subprocess.html#subprocess.Popen.returncode

In order to properly handle signals, we should change this to check `retcode != 0` instead.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #13692 from JoshRosen/dev-run-tests-return-code-handling.