RFR (xxs, test): 8146889: Update @requires expression for GC tests to run if GC is default

[Dmitry Fazunenko]

Hello,

may I have a couple reviews of a trivial change enabling two tests when 
no GC is explicitly specified.

http://cr.openjdk.java.net/~dfazunen/8146889/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8146889

Tested locally.

Thanks,
Dima