TestZKMainServer#testCommandLineWorks won't pass with default 4lw whitelist

Recent versions of ZooKeeper whitelist the so-called 4-letter word admin commands, and 'stat' is not in the default whitelist, so TestZKMainServer#testCommandLineWorks cannot get off the ground.. Set system property zookeeper.4lw.commands.whitelist=* in MiniZooKeeperCluster#setupTestEnv as we do not need to whitelist 4-letter commands for unit tests.