.gitlab-ci.yml: save rootfs as artifact for runtime tests

Sometimes when a test fails in a gitlab pipeline the reason of the
failure cannot be determined using only the logfile.

Add the modified rootfs as an artifact of the job to improve
troubleshooting. To accomplish this, always use -k option from the test
infra, unconditionally add the resulting images to the artifacts, and
let the runner do the cleanup for us.

These artifacts can also be useful when a test fails locally but pass at
gitlab runners.

When the test does not generate a image, this message is displayed in
the runner log:
WARNING: test-output/*/images/*: no matching files

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>