Make slow-stress tests run faster by running fewer VM variants.

Rubber stamped by Geoffrey Garen.

Tools: 

* Scripts/run-javascriptcore-tests:
* Scripts/run-jsc-stress-tests:

LayoutTests: 

* jsc-layout-tests.yaml:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@167191 268f45cc-cd09-0410-ab3c-d52691b4dbfc