PIG-4168: Initial implementation of unit tests (liyunzhang via praveen)

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/spark@1635107 13f79535-47bb-0310-9956-ffa450edef68
PIG-4168: Missed committing spark-tests file earlier (liyunzhang via praveen)

git-svn-id: https://svn.apache.org/repos/asf/pig/branches/spark@1635109 13f79535-47bb-0310-9956-ffa450edef68