Add arity checks to mjsunit's assertEquals and assertSame

BUG=None
LOG=n
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/309173003

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21748 ce2b1a6d-e550-0410-aec6-3dcde31c8c00