test-in-lxc.sh: Added script to perform per-commit testing with LXC.

This mimics test-in-vagrant.sh, but using an LXC container instead of a
Vagrant/Virtualbox virtual machine.

The tests run once the vm/container is up are identical to the ones run
by test-in-vagrant, they're only run using a different command. A good
to-do would be to extract that "inner" set of tests so it can be reused
independent of the container/VM used.

Testing under LXC is not much faster than Vagrant, since the bulk of the
time is spent in the provisioning phase (downloading + installing
packages) which doesn't benefit from LXC's faster creation and startup
times, but a crucial advantage is that it should be lighter on resources
and run on a cloud machine instance without nested hardware
virtualization support.

A disadvantage is that LXC commands need to be run with sudo, which
requires a bit more configuration than Vagrant. Required sudo
configuration is described at the top of the script.