RFR (S) 8223447: Stabilize gc/shenandoah/TestStringDedupStress test

[Aleksey Shipilev]

Test bug:
  https://bugs.openjdk.java.net/browse/JDK-8223447

I find that test failing with OOME almost reliably on x86_32. There are several things that are done
to stabilize the test: increased heap size, made sure the termination is time-based rather than
overwrite-iterations-based (which unties test configuration from heap size), pre-sized the hashmap,
added GC logging to simplify future investigations.

Webrev:
  http://cr.openjdk.java.net/~shade/8223447/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing test logs to see that dedup happened in most modes

-- 
Thanks,
-Aleksey