RFR (XS) 8215329: Modify ZGC requirement for HeapMonitorThreadTest.java

[JC Beyler]

Hi all,

When working on another webrev, I saw this problem:

Webrev: http://cr.openjdk.java.net/~jcbeyler/8215329/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8215329

(Basically, from what I understood from an email from Per Liden:
   - @requires !vm.gc.Z -> ZGC is built in the JDK
   - @requires vm.gc != "Z" -> ZGC is being used for the runtime
)

Thanks,
Jc