Let the caller define JSC_timeout for run-javascriptcore-tests
https://bugs.webkit.org/show_bug.cgi?id=136068

Patch by Akos Kiss <akiss@inf.u-szeged.hu> on 2014-08-19
Reviewed by Csaba Osztrogonác.

Keep the 60 second timeout for jsc tests as the default setting but
don't override the value of JSC_timeout if already set.

* Scripts/run-javascriptcore-tests:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172751 268f45cc-cd09-0410-ab3c-d52691b4dbfc