RFR: 8203923: Add @requires feature to check flag values for the running JVM

[Stefan Karlsson]

Hi all,

Please review this patch to add @requires vm.opt.final.<vm flag name>, 
as a way to filter jtreg tests on the final value of flags in the test VM.

http://cr.openjdk.java.net/~stefank/8203923/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8203923

This patch is needed to allow tests to be filtered out when ZGC is run.

ZGC doesn't support UseCompressedOops (and currently doesn't support 
ClassUnloading). Instead of adding @requires vm.gc != "Z" for tests that 
rely on ClassUnloading, I think it's nicer to write @requires 
vm.opt.final.ClassUnloading.

This will filter out the test if any of the following conditions are met:
1) The currently executing VM doesn't support ClassUnloading
2) -XX:-ClassUnloading is passed to the test

The second case can today be handled by @requires vm.opt.ClassUnloading 
== NULL | vm.opt.ClassUnloading == true, or maybe simpler @requires 
vm.opt.ClassUnloading != false, but if you need both @requires 
vm.opt.final.ClassUnloading should be used.

The current solution requires the queried flags to be listed in 
vmOptFinalFlags. Maybe there's a way to get this to cover all flags 
automatically?

Thanks,
StefanK