CI: Fix upload of screenshots in gitlab:ui:visual

Before this commit uploading of screenshots failed with

    WARNING: tests/__image_snapshots__/: no matching files
    ERROR: No files to upload
    ERROR: Job failed: exit code 1

After this commit it works again

    Uploading artifacts...
    gitlab-ui/tests/__image_snapshots__/: found 90 matching files
    Uploading artifacts to coordinator... ok

.gitlab/ci/frontend.gitlab-ci.yml

@@ -107,7 +107,7 @@ gitlab:ui:visual:
       - $CI_COMMIT_MESSAGE =~ /\[skip visual\]/i
   artifacts:
     paths:
-      - tests/__image_snapshots__/
+      - gitlab-ui/tests/__image_snapshots__/
     when: always
 
 karma: