Integration-test Arti in a Shadow network

Use shadow to create integration tests. This is orthogonal to creating integration tests in chutney, as the two have different tradeoffs. Shadow is a bit less "real" in that software communicates with a user-space "kernel". However this also allows it to be deterministic, and to decouple simulation-time from real-time. e.g. for small simulations we can run faster than real time.