integration: simple agent integration test

This adds a very basic integration test of the agent and supervisor.

`run.sh` will:
- set up a busybox chroot and install the harpoon components and dependencies.
- set up a busybox artifact for testing containers
- start the harpoon-agent in a container
- run any defined tests

/cc @peterbourgon @jmyounker 
Why is the enclosing directory called '_integration' rather than just 'integration', in the same way that 'misc' is not '_misc'?