RFR 8046869: Several java/lang/instrument/PremainClass/* tests fail due to timeout

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8046869
Webrev: http://cr.openjdk.java.net/~jbachorik/8046869/webrev.00

These two tests are timing out because they are deliberately crashing 
the virtual machine. This crash used to lead to coredump generation and 
the tests used to time out while dumping the core.

The fix is to add '-XX:-CreateCoredumpOnCrash' VM option to suppress the 
core generation.

Thanks,

-JB-