CI: Use mutter for the x11-full testsuite run

This spawns a mutter WM for the xvfb instance and wraps it in its own
dbus session.

Also use a larger screen size (1080p) - mostly so we can check the
testuite runs with differently sized screens, but alspo because that's
today's default screen size.

This exposes the issue from #4136 in the testsuite.

.gitlab-ci/run-tests.sh

@@ -13,7 +13,9 @@ export G_SLICE=always-malloc
 
 case "${backend}" in
   x11full)
-    xvfb-run -a -s "-screen 0 1024x768x24 -noreset" \
+    xvfb-run -a -s "-screen 0 1920x1080x24 -noreset" \
+          dbus-run-session \
+              mutter --x11 -- \
                   meson test -C ${builddir} \
                         --timeout-multiplier "${MESON_TEST_TIMEOUT_MULTIPLIER}" \
                         --print-errorlogs \