Add fast-variants feature to test driver.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/704723002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25139 ce2b1a6d-e550-0410-aec6-3dcde31c8c00