Update upload artifact action to version 3

We were getting a warning with version 2:

The following actions uses node12 which is deprecated and will be forced
to run on node16: actions/upload-artifact@v2. For more info:
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

.github/workflows/tests.yml

@@ -69,7 +69,7 @@ jobs:
           parallel: true
       - name: Upload screenshots
         if: failure()
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v3
         with:
           name: screenshots
           path: tmp/screenshots