Fix remote metastore tests which use ZooKeeper

Per vgarg's comment on HIVE-20794 at https://issues.apache.org/jira/browse/HIVE-20794?focusedCommentId=16714093&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16714093, the remote metatstore tests using ZooKeeper are flaky. They are failing with error "Got exception: org.apache.zookeeper.KeeperException$NoNodeException KeeperErrorCode = NoNode for /hs2mszktest".

Both of these tests are using the same root namespace and hence the reason for this failure could be that the root namespace becomes unavailable to one test when the other drops it. The drop seems to be happening automatically through TestingServer code.