JDK-7120365 DiffHBTest.java fails due to ConcurrentModificationException

[shanliang]

webrev: http://cr.openjdk.java.net/~sjiang/JDK-7120365/webrev.00/

The test is correct, it was implemented to verify the fix for bug 
4911721, but in addition it detects luckily another problem within the 
method ServerNotifForwarder.snoopOnUnregister

The problem was that during a "for" cycle, the set list was modified by 
anther thread, the solution is to use a unmodifiable set.

This fix is important, it not only re-enables a test, but also fixes a 
concurrent modification bug in the jmx notification.

Shanliang