RFR: 8204168: Increase small heap sizes in tests to accommodate ZGC

[Stefan Karlsson]

Hi all,

Please review this patch to increase the heap size for tests that sets a 
small heap size.

http://cr.openjdk.java.net/~stefank/8204168/webrev.01
https://bugs.openjdk.java.net/browse/JDK-8204168

This change is needed to test ZGC with these tests. ZGC doesn't use 
compressed oops and has an allocation memory reserve for the GC thread, 
and hence have a higher smallest heap size compared to other GCs in the 
code base.

There are some alternatives to this patch that we could consider (but I 
prefer the suggested patch):

1) Disable these tests when running with ZGC

2) Split all these tests into two copies, one copy for ZGC and another 
for the other GCs.

I've been running all of these changes through tier{1,2,3} and most of 
them has been run in tier{4,5,6}.

Thanks,
StefanK