Clutter's backend code depends on MetaBackend now, which makes it
impossible to initialize without resorting to private mutter API.

Luckily we only need Clutter for interactive tests which are broken
anyway, as Clutter.main() and friends were removed a while ago.

So for now, get at least unit tests working again by simply the
unnecessary Clutter initialization.

GNOME/gnome-shell!691