Testing shell app

Create a test app that exposes a shell.  This may be useful for tests that need to perform a sequence of manipulations to a single ongoing job/container.

Along the way, add support for specifying a complete Job configuration to pkg/exec.
You don't need to use discoverd if you don't want to, you should be able to get the `ActiveJob` from the host with `GetJob` or similar I think, but it's about the same amount of work LOC-wise.