RFR(XS): 8170490: Convert TestBufferingOopClosure_test to GTest

[Kirill Zhaldybin]

Dear all,

Could you please review this fix for 8170490?

Pretty straight-forward conversion, friend class name was changed to 
correspond other friend classes used for testing, deleted some defines 
we do not need.

WebRev: http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8170490/webrev.00/
CR: https://bugs.openjdk.java.net/browse/JDK-8170490

Thank you.

Regards, Kirill