+1 LGTM. Thanks Andras Bokor for submitting the patch.
Initially I expected MiniDFSCluster to extend AbstractService just like MiniYARNCluster, but now I am ok with your patch. Just keep it simple until a real use case calls for it.
I think unit test testDualClusters is redundant because testClusterWithoutSystemProperties already proves cluster.getDataDirectory() == getProp(HDFS_MINIDFS_BASEDIR) + "/data". This unit test sets HDFS_MINIDFS_BASEDIR to 2 different values and brings up 2 clusters, of course they will have different data directory.