RFR 9: 8137313 : TreeTest.java intermittently fails with a timeout

[Roger Riggs]

Please review an enhancement to the test to help diagnose this failure 
in the ProcessHandle TreeTest.

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-treetest-8137313/
Issue:
   https://bugs.openjdk.java.net/browse/JDK-8137313

Thanks, Roger