Verified fake for client implementations to test against

We need a low-overhead fake implementation for external clients (such as https://github.com/praekelt/go-http-api which already has its own fake implementation we can use as a starting point) to test against.

We don't want these external clients to require all of vumi-go just to get the test double, so it probably needs to live in a subproject with its own setup.py and such.

The functional tests for the real implementation also need to run against the fake implementation to verify that the behaviour is identical across the two.
That would add the toolkit as a dependency, which would either pull in cyclone and friends (which we don't want) or require a separate package for fake API tools. I'm not opposed to the latter (although I'm not convinced that it's worth the cost of having another small package for things to depend on), but I'd rather leave it until we build a second verified fake and have a better idea of what the common bits are.