RFR(S): 8168341: Gtests on JSon produce an enormous amount of hardly understandable output to stdout

[Kirill Zhaldybin]

Dear all,

Could you please review this fix for 8168341?

I redirected output to stringStream instead of tty and appended it to an 
assert message in case of failure.

WebRev: http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8168341/webrev.00/
CR: https://bugs.openjdk.java.net/browse/JDK-8168341

Thank you.

Regards, Kirill