RFR(S): 8168492: Convert CollectedHeap_test to GTest

[Kirill Zhaldybin]

Dear all,

Could you please review this fix for 8168492?

WebRev: http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8168492/webrev.00/
CR: https://bugs.openjdk.java.net/browse/JDK-8168492

Changes:
1. Changed _reserved to reserved_region() since _reserved is private but 
we already had an accessor.
2. Added more detailed messages for sanity checks
3. Otherwise pretty straight-forward conversion to gtest.

Thank you.

Regards, Kirill