Add Gitlab-CI

https://phabricator.collabora.com/T15904

This branch includes !11 (merged), !13 (merged), !14 (merged) (you might want to review those first), and then:

• d/tests: Log debug messages

• d/tests: Mark gnome-desktop-testing to be skipped if not installable

  If gnome-desktop-testing isn't available, we want to treat that as though this test had been skipped, rather than as though it had failed. d/tests/run effectively duplicates it anyway.

• debian: Enable Gitlab-CI

  We are targeting scout, but because scout is so old, its SDK environment is too old to run deb-build-snapshot. Instead, we do that step in a Debian 10 'buster' environment, and hand over the artifacts from that to the autopkgtest step.

  Our build-dependencies are not all preinstalled in the CI Docker images, so add them.

Requires !11 (merged), !13 (merged), !14 (merged), steam-ci-pipeline!4.