RFR: Make ReferenceCAS test more aggressive

[Roman Kennke]

This does the following to ReferenceCAS testcase:
- Make it use -XX:ShenandoahGCHeuristics=aggressive
- Use new string objects rather than constants
- verify that CAS does not destroy cmp values (yes, I have seen
this...)

http://cr.openjdk.java.net/~rkennke/refcas/webrev.00/

Ok?

Roman