Provide a dummy task in CI for the whole suite

This task depends on the matrix tests so we can set it as a required
task, rather than listing the whole matrix individually.

.github/workflows/ci.yaml

@@ -47,3 +47,10 @@ jobs:
       - name: Run tests
         run: |
           bundle exec rake test test:acceptance
+
+  suite:
+    name: Test suite
+    needs: test
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo Test suite completed