Check formatting in CI

Author: jelmer

.github/workflows/test.yml

@@ -41,6 +41,10 @@ jobs:
         run: |
           python -m ruff check .
 
+      - name: Format
+        run: |
+          python -m ruff format --check .
+
       - name: Tests
         run: |
           python -W once -m testtools.run testtools.tests.test_suite