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

[Roman Kennke]

It looks good to me.

Roman

> 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
>