Make TestFileTruncate more reliable

https://builds.apache.org/job/Hadoop-Hdfs-trunk/2163/testReport/junit/org.apache.hadoop.hdfs.server.namenode/TestFileTruncate/testTruncateWithDataNodesRestart/

org.apache.hadoop.ipc.RemoteException: The directory /test cannot be deleted since /test is snapshottable and already has snapshots
	at org.apache.hadoop.hdfs.server.namenode.FSDirSnapshotOp.checkSnapshot(FSDirSnapshotOp.java:226)
	at org.apache.hadoop.hdfs.server.namenode.FSDirDeleteOp.delete(FSDirDeleteOp.java:54)
	at org.apache.hadoop.hdfs.server.namenode.FSDirDeleteOp.deleteInternal(FSDirDeleteOp.java:177)
	at org.apache.hadoop.hdfs.server.namenode.FSDirDeleteOp.delete(FSDirDeleteOp.java:104)
	at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.delete(FSNamesystem.java:3046)
	at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.delete(NameNodeRpcServer.java:939)
	at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.delete(ClientNamenodeProtocolServerSideTranslatorPB.java:608)
	at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:636)
	at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:976)
	at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2172)
	at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2168)
	at java.security.AccessController.doPrivileged(Native Method)
	at javax.security.auth.Subject.doAs(Subject.java:415)
	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1666)
	at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2166)

	at org.apache.hadoop.ipc.Client.call(Client.java:1440)
	at org.apache.hadoop.ipc.Client.call(Client.java:1371)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:229)
	at com.sun.proxy.$Proxy22.delete(Unknown Source)
	at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolTranslatorPB.delete(ClientNamenodeProtocolTranslatorPB.java:540)
	at sun.reflect.GeneratedMethodAccessor21.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:186)
	at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:101)
	at com.sun.proxy.$Proxy23.delete(Unknown Source)
	at org.apache.hadoop.hdfs.DFSClient.delete(DFSClient.java:1711)
	at org.apache.hadoop.hdfs.DistributedFileSystem$14.doCall(DistributedFileSystem.java:718)
	at org.apache.hadoop.hdfs.DistributedFileSystem$14.doCall(DistributedFileSystem.java:714)
	at org.apache.hadoop.fs.FileSystemLinkResolver.resolve(FileSystemLinkResolver.java:81)
	at org.apache.hadoop.hdfs.DistributedFileSystem.delete(DistributedFileSystem.java:714)
	at org.apache.hadoop.hdfs.server.namenode.TestFileTruncate.setup(TestFileTruncate.java:119)