RFR(S): 8132711: Add tests which check that Humongous objects behave as expected after Mixed GC

[Kirill Zhaldbybin]

Dear all,

Could you please review this fix for 8132711?

This fix adds Mixed GC scenario to 
gc/g1/humongousObjects/objectGraphTest/TestObjectGraphAfterGC.java which 
creates different types of object graphs with Object arrays as nodes (to 
make them humongous) and checks that after different types GCs objects' 
graphs were handled correctly.

During Mixed GC scenario the test expects that all weakly reachable 
nodes should be collected since non-humongous nodes were previously 
promoted to old gen and several gc options were specified to guarantee 
that mixed gc collects all weakly unreachable objects in old gen.
Softly reachable nodes should not be collected.

WebRev: http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8132711/webrev.00/
CR: https://bugs.openjdk.java.net/browse/JDK-8132711

Thank you.

Regards, Kirill