RFR 8207964: [TESTBUG] Change stressTime to default to 30 for nsk tests

[Harold David Seigel]

Hi,

Please review this fix to change the default stress time for hotspot 
vmTestbase tests from 60 seconds to 30 seconds.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8207964/webrev/index.html

JBS Bug:  https://bugs.openjdk.java.net/browse/JDK-8207964

The fix was tested by running Mach5 hotspot tiers 1-5 on Linux-x64, 
Windows, Solaris, and Mac OS X.

Thanks, Harold