RFR: 8257876: Avoid Reference.isEnqueued in tests

[Kim Barrett]

Please review this change that eliminates the use of Reference.isEnqueued by
tests.  There were three tests using it:

vmTestbase/gc/gctests/ReferencesGC/ReferencesGC.java
vmTestbase/gc/gctests/WeakReferenceGC/WeakReferenceGC.java
jdk/java/lang/ref/ReferenceEnqueue.java

In each of them, some combination of using Reference.refersTo and
ReferenceQueue.remove with a timeout were used to eliminate the use of
Reference.isEnqueued.

I also cleaned up ReferencesGC.java in various respects.  It contained
several bits of dead code, and the failure checks were made stronger.

Testing:
mach5 tier1
Locally (linux-x64) ran all three tests with each GC (including Shenandoah).

-------------

Commit messages:
 - update WeakReferenceGC test
 - update ReferenceQueue test
 - update ReferencesGC test

Changes: https://git.openjdk.java.net/jdk/pull/1691/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1691&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257876
  Stats: 102 lines in 3 files changed: 21 ins; 39 del; 42 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1691.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1691/head:pull/1691

PR: https://git.openjdk.java.net/jdk/pull/1691