RFR(XS): 8008188: Add regression test for 8005875

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers review the regression test for 8005875 
- the webrev can be found at: 
http://cr.openjdk.java.net/~johnc/8008188/webrev.0/

The test is very simple and issues "jcmd <pid> Thread.print" against 
itself. With G1 and PGCT=0, and before the fix for 8005875, this command 
crashes the VM.

Testing:
jdk8 build (b76) with fix for 8005875; jdk8 build (b71) without fix for 
8005875; Changed the test options to run the test with the invalid flag 
-XX:+UseG2GC.

Thanks,

JohnC