Clean shared state pollution to avoid flaky tests.

hbase-server/src/main/java/org/apache/hadoop/hbase/monitoring/TaskMonitor.java

@@ -243,6 +243,11 @@ public class TaskMonitor {
     }
   }
 
+  public synchronized void purgeAllTasks() {
+    tasks.clear();
+    rpcTasks.clear();
+  }
+
   /**
    * This class encapsulates an object as well as a weak reference to a proxy
    * that passes through calls to that object. In art form:

hbase-server/src/test/java/org/apache/hadoop/hbase/monitoring/TestTaskMonitor.java

@@ -142,6 +142,7 @@ public class TestTaskMonitor {
     }
     assertEquals("RPC Tasks have been purged!", RPCTaskNums, remainRPCTask);
     tm.shutdown();
+    tm.purgeAllTasks();
   }
 
   @Test