workflows: Add a "all tests pass" check

This way we can avoid naming all the matrix tests individually
in "required checks to pass before merging" in GitHub UI (which
requires tweaking everytime supported Python versions change).

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

.github/workflows/_test.yml

@@ -77,6 +77,13 @@ jobs:
         run: |
           coveralls --service=github
 
+  all-tests-pass:
+    name: All tests passed
+    needs: [lint-test, tests]
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo "All test jobs have completed successfully."
+
   coveralls-fin:
     # Always run when all 'tests' jobs have finished even if they failed
     # TODO: Replace always() with a 'at least one job succeeded' expression