RFR: Readjust ShenandoahStrDedupStress test to avoid timeout

[Zhengyu Gu]

ShenandoahStrDedupStress test has caught quite a few hard to find bugs, 
cause it *really* stress GC subsystem, but it also prone to timeout.

This patch takes GC cycles into according during string rewriting phase, 
make sure that there are enough cycles to exercise deduplication code, 
but don't have to prolong the tests.


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


fastdebug test on polwarth:

Before:
real	11m23.770s
user	56m17.319s
sys	0m26.255s

After:
real	5m52.861s
user	34m24.452s
sys	0m16.774s


Thanks,

-Zhengyu