RFR 9: 8085981 java/lang/ProcessHandle/OnExitTest.java: AssertionError: Child onExit not called

[Roger Riggs]

Please review this test fix for a race condition checking that the 
onExit CompletableFuture
was completed and with the correct value.

Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-onexit-8085981/
Issue:
https://bugs.openjdk.java.net/browse/JDK-8085981

Thanks, Roger