RFR 8029890: java/lang/management/ThreadMXBean/ThreadBlockedCount.java fails: Blocked thread has 4 blocked counts. Expected 3

[Jaroslav Bachorik]

Please, review the following test fix.

Issue : https://bugs.openjdk.java.net/browse/JDK-8029890
Webrev: http://cr.openjdk.java.net/~jbachorik/8029809/webrev.00

The test fails intermittently due to ThreadMXBean.getThreadInfo() 
blocking the tested thread from time to time. The solution is not to 
invoke this method from within the tested thread and, in order to make 
the test more readable and maintainable, replace the 
ThreadExecutionSynchronizer with the standard j.u.c.Phaser.

Thanks,

-JB-