Mark slow mozilla test.

This test is the remaining low hanging fruit in quick check and blocks the runner for 5 extra seconds.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/281153002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21314 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'