CI: add testing and testimage

Add a testing task to the gitlab CI and run testimage in that job.
Currently, it only runs against some of the qemu based machines.
Using slirp, there is no need for a privileged container or tun/tap
devices.

Change-Id: Ia85a3d0089f7d4dc7595c3a45d328c79d8e675f1
Signed-off-by: Jon Mason <jon.mason@arm.com>