worker: add integration tests
worker: add integration tests

Previously, `backend` was being exercised only with mocks. While that's great
for the purpose of unit testing our expectations, it does not validate whether
with a real daemon (containerd) our expectatons match the reality.

The tests added here focus solely on interacting with containerd via the
backend, using the real containred client implemntation (libcontainerd.client),
ensuring that we're able to go through the expected workflow:

	1. starting the client & pinging containerd

	2. creating a container

	3. listing containers (getting ours back)

	4. running a process within the container

	5. destroying it

Naturally, these simply fail at the moment :sweat_smile: the backend
functionality for creating, listing and destroying still needs to be
implemented.

To run these, make sure you have `containerd` *already running*, and that you
have access to the socket (by default you need to be uid=0).

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse