RFR: 8178813: Add test for G1 pre-barrier on dereference of weak JNI handles

[Kim Barrett]

Still looking for reviewers.

> On Apr 14, 2017, at 4:41 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this new test of the handling of weak JNI handles during
> G1 concurrent mark.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8178813
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8178813/hotspot.00/
> 
> Testing:
> JPRT  
> 
> Locally ran new test with G1-specific code in JNIHandles::resolve and
> MacroAssembler::resolve_jobject temporarily removed, and verified
> expected failures.