Add `fulfill` event to listen for when the `plan` count is met.

I want to test several actions (say, 5 of them) against a single http server, but I don't want to nest them and I don't really need complex async semaphore. So I set `t.plan(5)` and then send off each of my requests, each of which have an async callback doing something like `t.equal("Something", res.body)`. I want to catch when those are all done, so that I can run `server.close()` and return. This event, `fulfill`, is triggered kind of in between the last `result` event and the `end` event (which doesn't occur until the child process ends).