Relax test constraints from r15439 (Join threads after stopping).

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/18509002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15440 ce2b1a6d-e550-0410-aec6-3dcde31c8c00