RFR: 8241828: JFR: Some streaming tests require a larger heap size with ZGC

[Stefan Karlsson]

Hi all,

Please review this small patch to increase the max heap size of a couple 
of JFR streaming tests.

https://cr.openjdk.java.net/~stefank/8241828/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8241828

The default heap size is 512m when these tests are run. G1 uses almost 
500m, but ZGC needs a bit more. I propose that we set the max heap size 
to 768m.

Thanks,
StefanK