support/testing: add runtime testing for read-only systemd

We add the 3 following combinations:

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

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>
[Arnout: regenerate .gitlab-ci.yml]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>