Manual DRM-backend smoke-test with VKMS

Add a test in the test suite that runs Weston/DRM/pixman using the non-default seat seat-vkms. Whether this actually runs depends on whether there is a DRM device available on seat-vkms, otherwise the test is skipped. Input devices are not required or used.

I call this a manual test, because this test would depend on the local system setup to have VKMS kernel module loaded and seat-vkms configured.

It requires Weston to be able to start on a non-default seat with normal user permissions: either the user must have access to the VKMS DRM device, or logind needs to be able to hand that out. weston-launch cannot be used, because it would require root permissions to install as setuid-root, and without setuid-root it cannot provide anything useful.

Weston on non-default seat depends also on skipping all VT/tty setup.

This task depends on !287 (merged).