Add support for pit.only

Add support for pit.only; to match it.only().

Also, a common mistake is to forget to return a Promise. Currently the test would just silently pass without actually testing anything. Fail loudly instead.
I think there are cases where I just used `pit()` everywhere in a spec just for consistency -- even for non-promise tests (in Jest's own tests, for example). When we update to this version, I guess we'll just need to make sure to go through and fix those up -- I think this is a good thing, and I probably shouldn't have done that.