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.
I don't think it'll help much in practice.  On the whole, this is definitely not meant to be used for high-fidelity lossless communication -- it's already making a ton of simplying assumptions like relatively trusted input, throwing together stderr and stdout is fine, there is no stdin, etc.  I think the consuming test code is almost invariably going to be asserting against a string that covers all corners anyway; if something in the command does something unexpected, the test will already fail.  If there is no discriminating output, one could just suffix `; echo $?` to the input.

We could also put exit codes and such in response headers, but I think that would result in a helper method to parse it... so, long story short, I guess the question is do we care enough to build this testing hack into a full shrinkwrapped RPC service?