RFR 6987597: ManagementFactory.getGarbageCollectorMXBeans() returns empty list with CMS

[Jaroslav Bachorik]

Please, review this test fix.

Issue : https://bugs.openjdk.java.net/browse/JDK-6987597
Webrev: http://cr.openjdk.java.net/~jbachorik/6987597/webrev.03

The fix moves the execution of the test routine to the 
GarbageCollectorMXBean notification handler - at that moment it's 
certain that GC has already happened and it is safe to assert the data 
provided by the MBean.

Thanks,

-JB-