CI: store test images in artifacts

If a screenshooting test fails, it quite likely writes not only the screenshot
but also a diff image highlighting the failed pixels. These would be good to
have in the CI artifacts for postmortem.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

.gitlab-ci.yml

@@ -75,6 +75,7 @@ container_prep:
     name: weston-$CI_COMMIT_SHA
     when: always
     paths:
+    - build-*/*.png
     - build-*/meson-logs
     - build-*/weston-virtme
     - prefix-*