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 
Folders / files prefixed with _ are ignored by go tooling; in this
case, that means "go test ./..." run from the harpoon root will not
attempt to run the tests under _integration.