RFR[9]: 8178832: (ref) jdk.lang.ref.disableClearBeforeEnqueue property is ignored

[Kim Barrett]

Please review this fix for a bug in JDK-8175797: the mechanism
provided by that change for reverting to the old behavior of
Reference.enqueue (for backward compatibility) does not work.

Also fixed the ReferenceEnqueue test changes also made as part of
JDK-8175797.  That test would havecaught the problem with the property
but for bugs in the test.

CR:
https://bugs.openjdk.java.net/browse/JDK-8178832

Webrev:
http://cr.openjdk.java.net/~kbarrett/8178832/jdk00/

Testing:
Ran fixed test with unfixed enqueue, and verified expected failure.
Ran fixed test with fixed enqueue, and verified expected pass.