gitlab CI: add support for DRM-backend tests

gitlab CI: add support for DRM-backend tests

In order 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.
This patch add support to run the tests using VKMS (virtual KMS).
To achieve this, virtualization is needed, as we need to run a
custom kernel during the CI job. We've decided to go with virtme,
as it is simpler to setup and works good for our use case.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>