I've got bad news and good news: There is no way to mock HTTP responses for Runtime yet, but there will be very soon.
In the next version of Runtime SDK for .NET, we plan to include an easy way to replace HttpMessageHandler for any request issued by Runtime. This will allow integration with tools like MockHTTP. This will also allow for scenarios like reading responses from local files (e.g. something like this) or generating them on the fly.
I will post another reply when this functionality is released, and I'll have some sample code to share at that time.