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.

.gitlab-ci/Dockerfile

@@ -70,4 +70,4 @@ RUN useradd -u $HOST_USER_ID -ms /bin/bash user
 USER user
 WORKDIR /home/user
 
-ENV LANG C.utf8
+ENV LANG C.UTF-8

.gitlab-ci/test-docker.sh

@@ -19,6 +19,5 @@ xvfb-run -a -s "-screen 0 1024x768x24" \
     meson test \
         --print-errorlogs \
         --suite=gtk+ \
-        --no-suite=gtk+:gdk \
         --no-suite=gtk+:gsk \
         --no-suite=gtk+:a11y