RFR(S): 8132723, 8132724: Add tests which check that soft/weak references to humongous objects should work correctly

[Kirill Zhaldybin]

Hi!

Could you please review the following patch that adds a test that checks 
that soft/weak references to humongous objects work correctly?

The test checks that weak and soft references behave correspondingly
to actual object behavior:
1. If an object is collected a weak/soft reference .get() should
return null.
2. If an object is not collected a weak/soft reference .get() should
not return null.

Two White Box API methods were added:
1. g1BelongsToHumongousRegion which checks that address belongs to
humongous region
2. g1BelongsToFreeRegion which checks that address belongs to free region
These methods do not  impact G1 performance/stability since they don't 
change anything in G1 execution.

The only used method from gc/testlibrary is waitTillCMCFinished, which 
is under review here -
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2015-November/015643.html

CR: https://bugs.openjdk.java.net/browse/JDK-8132723
     https://bugs.openjdk.java.net/browse/JDK-8132724

Webrev:
     main: 
http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8132724/webrev.02/main/
     hotspot: 
http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8132724/webrev.02/hotspot/

Thank you.

Regards, Kirill