RFR 8031701: java/lang/management/ThreadMXBean/Locks.java: Thread WaitingThread is expected to wait on Object but got null Thread.State = RUNNABLE

[Jaroslav Bachorik]

Please, review the following test fix.

Issue : https://bugs.openjdk.java.net/browse/JDK-8031701
Webrev: http://cr.openjdk.java.net/~jbachorik/8031701/webrev.00
The ThreadExecutionSynchronizer was replaced by Phaser - this should 
give more predictable results.
Also, "checkBlockedObject()" failed to re-retrieve the ThreadInfo while 
waiting for the thread to become blocked. This would lead to timing out 
the test and producing the error message used as the issue subject. The 
fix for this is at L78-79

Thanks,

-JB-