introduce the concept of backfire

The idea here is to have a strategy for moving away from integration tests. When mocking is done via cassette as it is currently designed, we are left with a lot of work to pass the tests each time we change what is being requested from a dependent service to another service. By using "backfire", we can start to slowly and methodically remove HTTP dependencies from our tests, and move towards a unit-test only mindset.

At the end of the day, the data coming over the wire must be allowed to be malleable. We can't code our tests against old data.
Also, can you think of a more explicit name? Like `ensure_no_http_request`?