RFR: 8235250: Create (test) abstraction for allocating objects that is immune to removal by optimizations

[Leo Korinth]

Hi,

Please review this change to some tests that is needed for future 
optimizations. Reference.reachabilityFence seems not to be usable for 
ensuring GC allocations.

I have added a new abstraction Allocation.blackHole(Object obj) for this.

CR:
https://bugs.openjdk.java.net/browse/JDK-8235250

Webrev:
http://cr.openjdk.java.net/~lkorinth/8235250/00

Testing:
currently running tier 1-3 on linux

Thanks,
Leo