Add randomized test generation capability to tools/generate-runtime-tests.py

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/288443005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21307 ce2b1a6d-e550-0410-aec6-3dcde31c8c00