Exit HBaseTestDataRegionAssignment forcefully after success to avoid HDFS-6057.

Change-Id: I125c09c0bf817f3427302adf89e17c419200907c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1768
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins