[Flakey Tests] branch-2 TestMasterNoCluster.testStopDuringStart

org.apache.hadoop.hbase.master.TestMasterNoCluster.testStopDuringStart is (only) flakey on branch-2 currently. Fails here:

Error Message

KeeperErrorCode = Directory not empty for /hbase/backup-masters

Stacktrace

org.apache.zookeeper.KeeperException$NotEmptyException: KeeperErrorCode = Directory not empty for /hbase/backup-masters
at org.apache.hadoop.hbase.master.TestMasterNoCluster.tearDown(TestMasterNoCluster.java:121)

I can see the zk events in teardown as we purge children as part of cleanup. Can also see that the backup master registers later. Other than that, log is opaque on why the teardown is failing. This is just clean up so adding in retry to see if that helps.