Skip mjsunit/migrations test in debug mode to avoid timeout failures.

TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/364663002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22133 ce2b1a6d-e550-0410-aec6-3dcde31c8c00