Make d8-os test less flaky.

Make d8-os test less flaky.

Because of stress testing there may be parallel instances of the test
running, which makes it fail when using a hardcoded test dir
name. Adding a random suffix helps. It'd be nicer to add something
like pid, but there doesn't seem to be an easy way of getting it.

R=yangguo@chromium.org

Review URL: http://codereview.chromium.org/7491057

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8848 ce2b1a6d-e550-0410-aec6-3dcde31c8c00