RFR: Shenandoah filter for gc/TestFullGCCount.java test

[Zhengyu Gu]

Shenandoah, by default, sets ExplicitGCInvokesConcurrent = true, which 
means it runs concurrent GC for System.gc().

We need to filter out the test, unless ExplicitGCInvokesConcurrent is 
explicitly set to false.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/filter_conc_system_gc/webrev.00/

Test:

   make CONF=linux-x86_64-server-fastdebug run-test 
TEST=gc/TestFullGCCount.java 
JTREG="VM_OPTIONS=-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC"

and

make CONF=linux-x86_64-server-fastdebug run-test 
TEST=gc/TestFullGCCount.java 
JTREG="VM_OPTIONS=-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC 
-XX:-ExplicitGCInvokesConcurrent"


Thanks,

-Zhengyu