support/testing: add lxc test

The test starts a simple container with an iperf3 server.
The container is using the tini init system, with a shared rootfs.
An iperf3 client is started from the host to check that the container
is really up and running.

Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>