Run DRM-backend tests on GitLab CI

Hello! In !420 (merged) we have introduced support to run DRM-backend tests locally. Now we add support to run them on GitLab CI.

• 1/2: We run DRM-backend tests on non-default seat seat-weston-test, as it avoids unnecessarily opening input devices that may be present on default seat seat0. This will make logind fail, as we are trying to use a seat that is different from the one we are logged in, so we're going to rely on launcher-direct. This commit adds a check in launcher-direct to skip VT/tty management if user is running on a non-default seat, as VT/tty should be exposed only for the default seat.

• 2/2: To run DRM-backend tests, a DRM-device is needed. As we do not necessarily have control of the hardware that is going to run our tests in GitLab CI, DRM-backend tests were being skipped. Here we add the mechanism to run DRM-backend tests in GitLab CI. The first thing we do is add support to compile a custom Linux image with VKMS (virtual KMS) enabled, so we can make sure that we have a device to run the DRM-backend tests. Then we run this custom Linux image using virtme, which is a simplified way of running QEMU.