HDFS-5472. Fix TestDatanodeManager, TestSafeMode and TestNNThroughputBenchmark. (Contributed by szetszwo)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2832@1539736 13f79535-47bb-0310-9956-ffa450edef68