HDFS-4520. Support listing snapshots under a snapshottable directory using ls.  Contributed by Jing Zhao


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1449862 13f79535-47bb-0310-9956-ffa450edef68
Add the new test file by HDFS-4520.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1449863 13f79535-47bb-0310-9956-ffa450edef68