RFR (XS) 8247778: ZGC: More parallel gc/z/TestUncommit.java test configuration

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8247778

See the details in the RFE.

Fix:
  https://cr.openjdk.java.net/~shade/8247778/webrev.01/

Testing: affected test (and nothing else, because I consider the change trivial)

-- 
Thanks,
-Aleksey