HBASE-21203 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. Set system property
zookeeper.4lw.commands.whitelist=* in MiniZooKeeperCluster#setupTestEnv
as we do not need to whitelist 4-letter commands for unit tests.

hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/MiniZooKeeperCluster.java

@@ -176,6 +176,8 @@ public class MiniZooKeeperCluster {
     // set env and directly in order to handle static init/gc issues
     System.setProperty("zookeeper.preAllocSize", "100");
     FileTxnLog.setPreallocSize(100 * 1024);
+    // allow all 4 letter words
+    System.setProperty("zookeeper.4lw.commands.whitelist","*");
   }
 
   public int startup(File baseDir) throws IOException, InterruptedException {