In the latest version of Chrome Canary, a beta channel for the browser, a new featured was added to the DevTools responsive layout allowing you to easily create a screenshot, including a mobile frame.
An example on how to test a client which calls out to an external API, without requiring the API server to be up and running. From Testing Techniques video by Andrew Gerrand at Google I/O 2014.
Go has a standard library
net/http/httptest which you can use to ...