gitlab-ci: enable gtk+:gdk tests
xvfb doesn't like C.utf8 and returns XLocaleNotSupported.
While (afaik) C.utf8 and C.UTF.8 should be the same thing, and the former is returned by locale -a on Fedora, switch to C.UTF-8 to make xvfb happy.
This makes gtk+:gdk tests pass.