bakerytest: new package

This should make it simpler to test services that issue macaroons.

Also add httpbakery.DischargeAll to make it possible to use
the httpbakery discharge machinery without going through
httpbakery.Do.

Not an issue, but questions... Is this a `bakerytest` or `httpbakerytest` package? Will httpbakery get gopkg-versioned independently of bakery? If so, it seems like this package needs to get versioned along with httpbakery.