HDFS-6059. TestBlockReaderLocal fails if native library not available (ajisakaa via cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1575002 13f79535-47bb-0310-9956-ffa450edef68
Merge r1574999 from branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1575003 13f79535-47bb-0310-9956-ffa450edef68