RFR(S): 8203943: eventThreadGroup was null in TestJavaBlockedEvent.java

[Markus Gronlund]

Greetings,

Please review this solution to address the issue described in the bug.

Bug: https://bugs.openjdk.java.net/browse/JDK-8203943 
Webrev: http://cr.openjdk.java.net/~mgronlun/8203943/webrev00/ 

In addition, the JfrCheckpointThreadClosure now only needs a single iteration instead of the previous two.

Thanks
Markus