RFR 8078143: java/lang/management/ThreadMXBean/AllThreadIds.java fails intermittently

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8078143
Webrev: http://cr.openjdk.java.net/~jbachorik/8078143/webrev.00

The test fails intermittently due to inconsistent reporting of the live 
threads number.It is related to 
https://bugs.openjdk.java.net/browse/JDK-8021335 (or, better said, 
caused by) - certain performance counters used for the thread accounting 
are being updated under a mutex but are read without it - and it can 
lead to stale data being reported. More details are available in the 
linked issue and discussion.

Because of this it is not enough to join() a terminating thread to make 
sure the numbers would be correct. Luckily enough, it seems to be 
sufficient to wait for a short time before actually accessing the 
counters to be able to get a consistent view. In this fix I opted for a 
ridiculously huge interval of 500ms just to be sure.

Thanks,

-JB-