RFR(s): 8151283: Implement setting jtreg @requires property vm.isG1Supported.

[Dmitry Fazunenko]

Hello,

I'm looking for a couple of Reviewers for a simple change, which allows 
to skip tests depending on non supported garbage collectors.

http://cr.openjdk.java.net/~dfazunen/8151283/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8151283

The fix introduces four new requires boolean properties:
      vm.gc.G1.acceptable
      vm.gc.Serial.acceptable
      vm.gc.Parallel.acceptable
      vm.gc.ConcMarkSweep.acceptable
Which are set by the jtreg plugin prior the test execution starts.
After that fix we will be able to replace in tests:
    @requires vm.gc == null | vm.gc == "G1"
with
    @requires vm.gc.G1.acceptable

'vm.gc.G1.acceptable' will be evaluated to 'false' not only if other GC 
is set externally, but also if G1 is not supported.
In other words the fix allows not to run GC specific tests in not 
applicable configurations.

Thanks,
Dima