[SPARK-5437] Fix DriverSuite and SparkSubmitSuite timeout issues

In DriverSuite, we currently set a timeout of 60 seconds. If after this time the process has not terminated, we leak the process because we never destroy it.

In SparkSubmitSuite, we currently do not have a timeout so the test can hang indefinitely.

Author: Andrew Or <andrew@databricks.com>

Closes #4230 from andrewor14/fix-driver-suite and squashes the following commits:

f5c80fd [Andrew Or] Fix timeout behaviors in both suites
8092c36 [Andrew Or] Stop SparkContext after every individual test