RFR 8141591: javax/management/remote/mandatory/threads/ExecutorTest.java fails intermittently

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8141591
Webrev: http://cr.openjdk.java.net/~jbachorik/8141591/webrev.00

In rare circumstances, when an external executor is provided, the 
ClientNotifForwarder$NotifFetcher.doRun() method might fail because of 
RejectedExecutionException caused by the executor being externally shut 
down.

The patch adds a guard against this possibility. If the executor has 
been shut down the fetcher will also properly stop.

Thanks,

-JB-