Also return the requisites from check_requisite; add mod_watch support to states.test

I'd like to be able to get at the actual returns for watched states from a ``mod_watch`` function but we don't have enough detail in ``__low__`` to look them up in ``__running__`` (i.e. not enough data to call ``_gen_tag()``; at least not without repeating the ``check_requisite()`` work to get the full tag). Since we're already doing that work, how about we pass it through instead? The ``reqs`` dictionary that ``check_requisite()`` generates is crazy cool, I can see this being useful in other places as well.

@thatch45 thoughts?