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

[Roger Riggs]

Ping and note this change is simplified after the fix for 8085981.

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