RFR: Cap heap size for TestRegionSizeArgs test

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/tests-trgs/webrev.01/

On large machine that has enormous default heap size, TestRegionSizeArgs begins to timeout. This
happens for small heap region size settings which yields lots of regions. The startup sequence in
Shenandoah adds all these regions into the freeset *and* it checks contains() on them in assert,
which exhibits quadratic behavior.

Thanks,
-Aleksey