Don't parallelize tests that register extensions.

These tests register extensions on the isolate and the configuration of the extensions runs out of scope. If run in parallel, other tests access the isolate's state and read the registered extensions.

BUG=
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/149413005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18921 ce2b1a6d-e550-0410-aec6-3dcde31c8c00