Update upload artifact action to version 4

We were getting a warning with version 3:

Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20: actions/upload-artifact@v3. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/tests.yml

@@ -69,7 +69,7 @@ jobs:
           parallel: true
       - name: Upload screenshots
         if: failure()
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: screenshots
           path: tmp/screenshots