RFR: 8221913: Add GC.selected() jtreg-ext function

[Kim Barrett]

> On Apr 3, 2019, at 2:44 PM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi all,
> 
> Please review this patch to add a convenience function to get the selected GC in jtreg tests.
> 
> https://cr.openjdk.java.net/~stefank/8221913/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8221913
> 
> Thanks,
> StefanK

Looks good.