RFR (S): 8026043: add regression test for JDK-8000831

[Filipp Zhinkin]

Hi,

I would like to get couple review on test, that cover issue fixed by 
8000831 - Heap verification output incorrect/incomplete.

Test verifies that options VerifyBeforeGC and VerifyAfterGCemit 
appropriate output if they are turned on and never
emit corrupted output as it was before 8000831 fixed.

Bug ID:  https://bugs.openjdk.java.net/browse/JDK-8026043
Webrev:  http://cr.openjdk.java.net/~kshefov/8026047/webrev.00/
Testing: manual on local host, automated on various platforms.

Thanks,
Filipp.