Move unit tests to test/unittests.

As per discussion on the V8 team, this is the place we want them to live,
not following the Chrome Style Guide for this.

BUG=v8:3489
LOG=y
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/615393002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24350 ce2b1a6d-e550-0410-aec6-3dcde31c8c00