support/testing: add runtime testing for init systems

The "builtin" kernel does not boot a systemd-based system, so
we resort to building the same one as currently used by our
qemu_arm_vexpress_defconfig.

We test the 8 following combinations:

  - busybox, read-only, without network
  - busybox, read-only, with network
  - busybox, read-write, without network
  - busybox, read-write, with network

  - basic systemd, read-write, network w/ ifupdown
  - basic systemd, read-write, network w/ networkd
  - full systemd, read-write, network w/ networkd

  - no init system, read-only, without network

The tests just verify what the /sbin/init binary is, and that we were
able to grab an IP address. More tests can be added later, for example
to check each systemd features (journal, tmpfiles...)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
[Arnout: update .gitlab-ci.yml]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>