tester: Check for distributed mode in Spark test
tester: Check for distributed mode in Spark test

This commit updates the Spark integration tester to ensure that
the Spark job is running on the workers in the cluster, rather
than running in local mode on a single machine.

LuiseV/quilt,LuiseV/quilt,quilt/quilt,quilt/quilt,NetSys/quilt,LuiseV/quilt,kelda/kelda,NetSys/quilt,NetSys/quilt,NetSys/quilt,LuiseV/quilt,quilt/quilt,NetSys/quilt,LuiseV/quilt,kelda/kelda,quilt/quilt,kelda/kelda,kelda/kelda