I've extended Resty so that it's easy to mock responses. At the moment, it's very simple - given an HTTP method and a URL, we "send back" a static string response. That's it.
$resty = new \MockResty\MockResty();
// send back a json response
$resty->on('get','/version', json_encode(array('version'=>'0.99b')));
// change the response header to 403, but send back an empty body
$resty->on('get','/auth/forbidden', 'HTTP/1.1 403 Forbidden');
At present, MockResty is very dumb - it won't auto-json the responses, for example. It will simply return what has been set.