RFR 9: 8130296 [TESTBUG] java/lang/ProcessHandle/OnExitTest - Unaccounted for children

Please review this change to a ProcessHandle test to address intermittent failures. Unexpected subprocesses has been observed. The test succeeds if all of the processes under test are accounted for; others are ignored. Webrev: http://cr.openjdk.java.net/~rriggs/webrev-exit-8130296/ Issue: https://bugs.openjdk.java.net/browse/JDK-8130296 Thanks, Roger