RFR: 8257876: Avoid Reference.isEnqueued in tests [v2]

[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).

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  move REMOVE and RETAIN decls and init

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1691/files
  - new: https://git.openjdk.java.net/jdk/pull/1691/files/e87206a8..01710567

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1691&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1691&range=00-01

  Stats: 6 lines in 1 file changed: 4 ins; 2 del; 0 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