tests: Run tests as user ckan-sys

Necessary so that pytest coverage can save its database file in the /srv/app directory, which ckan-sys owns.

.github/workflows/test.yml

@@ -74,5 +74,5 @@ jobs:
         docker exec -u root test_ckan $WORKDIR/bin/install_test_requirements.sh
     - name: Run tests
       run: |
-        docker exec test_ckan pytest --ckan-ini=$WORKDIR/test.ini --disable-warnings --cov=ckanext.switzerland \
+        docker exec -u ckan-sys test_ckan pytest --ckan-ini=$WORKDIR/test.ini --disable-warnings --cov=ckanext.switzerland \
           $WORKDIR/ckanext/switzerland/tests