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

[Kim Barrett]

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.