[KYUUBI #395] [WIP] FIx flaky test in ConnectionLevelSparkEngineSuite

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### _Why are the changes needed?_
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### _How was this patch tested?_

Closes #395 from ulysses-you/flaky-test.

e8bdd4d [ulysses-you] empty
08febfc [ulysses-you] empty
c3805d1 [ulysses-you] empty
702ac2c [ulysses-you] empty
a544ef0 [ulysses-you] empty
0bd8878 [ulysses-you] empty
dbd53e4 [ulysses-you] fix
f4be895 [ulysses-you] empty
195933a [ulysses-you] empty
cb69e0d [ulysses-you] wip

Authored-by: ulysses-you <[email protected]>
Signed-off-by: ulysses-you <[email protected]>

Author: ulysses-you

kyuubi-ha/src/main/scala/org/apache/kyuubi/ha/client/EngineServiceDiscovery.scala

@@ -34,7 +34,8 @@ class EngineServiceDiscovery private(
   def this(server: Serverable) =
     this(classOf[EngineServiceDiscovery].getSimpleName, server)
 
-  override def stop(): Unit = {
+  override def stop(): Unit = synchronized {
+    closeServiceNode()
     conf.get(ENGINE_SHARED_LEVEL) match {
       // For connection level, we should clean up the namespace in zk in case the disk stress.
       case "CONNECTION" if namespace != null =>