RFR 8076344: serviceability/dcmd/vm/SetVMFlagTest.java test fails with "java.lang.Error: 'MaxHeapSize' flag is not available or immutable"

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8076344
Webrev: http://cr.openjdk.java.net/~jbachorik/8076344/webrev.00

Originally, the test used some well known flags to assert the correct 
functionality. I didn't expect eg. 'MaxHeapSize' flag to go away so 
quickly :)

The solution is to pick whichever mutable/immutable flag of a certain 
type and carry on the test using this flag. This way the test will 
always use only the available flags.

Thanks,

-JB-