How do I mock HTTP requests for PHPUnit?

Question

I'm writing a plugin that makes requests to the Facebook graph API. As I don't want my unit tests to actually make these requests, how would I overcome this? My method calls both wp_remote_get and wp_remote_post. Searching there does seem to be a way to mock functions using runkit, here

I want to avoid having contributers requiring too many dependencies so would like to avoid the above method. Is there any other options? My class extends the WP_UnitTestCase so I'm hoping maybe there's something from the wp unit-tests that I could use?