Add a nightly test workflow

.gitea/workflows/nightly_test.yml

@@ -0,0 +1,15 @@
+name: Run tests every night
+on:
+  schedule:
+    - cron: "05 17 * * *"
+
+jobs:
+  make-test:
+    name: Run make test
+    runs-on: [django-dav-events]
+    steps:
+      - name: "Checkout the repository"
+        run: git clone "${{ gitea.server_url }}/${{ gitea.repository }}" ./repository
+      - name: "Run make test"
+        working-directory: ./repository
+        run: make test