8062536: [TESTBUG] Conflicting GC combinations in jdk tests
Reviewed-by: brutisso, dholmes

8014890: (ref) Reference queues may return more entries than expected
Summary: When enqueuing references check whether the j.l.r.Reference has already been enqeued or removed in the lock. Do not enqueue them again. This occurs because multiple threads may try to enqueue the same j.l.r.Reference at the same time.
Reviewed-by: mchung, dholmes, plevart, shade
Contributed-by: thomas.schatzl@oracle.com