RFR: Refactoring ShenandoahStrDedupStress test to reduce test time

[Zhengyu Gu]

This refactoring reduces ShenandoahStreDedupStress fastdebug test time 
from ~24 minutes to about 4 minutes.

Of course, the test is less stress than it used to. It used to run > 50 
GC cycles, now down to 6 - 10 cycles for heuristics other than 
"aggressive", but enough cycles to trigger string deduplications.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/strdedup_stress/webrev.00/

Test:
   ShenandoahStrDedupStress test fastdebug and release.

Thanks,

-Zhengyu