TestDFSIOEn allow to configure output directory

TestDFSIO & TestDFSIOEn has a hardcoded location for its files to be written and read to or from. This location is '/benchmarks'. However it might have a problem if I don't have permission to write '/'(root) in HDFS. It'd be convenient to have a configuration option to specify an alternative location on demand.

This test was performed with hadoop 2.4.0 on Amazon EMR and hadoop 2.4.0 on my cluster.
I did not run the test on master branch(hadoop 1), it is presumed to be probably work.