The `waitUntilCalled` utility now has a timeout. It will now throw an
error if the stub is not called enough times, rather than blocking
forever.
The return type had to be changed to a function, so that we could throw
when the timeout is triggered. I tried returning an error that rejected
first, but if you don't handle the error synchronously Node.js will
consider it to be an unhandled Promise rejected (even if it _is_
handled later on).
I worked around this by resolving in the timeout case as well, so that
there is never a "deferred" Promise exception in the timeout case. The
returned function re-throws the error if it's given. That way there is
never any unhandled Promise rejection.
