github(workflows): add stages/test to the CI run too

Include the new `stages/test` test category to the CI runs as well.

Note that because `stages/__init__.py` and `stages/test/__init__.py`
are missing it is not possible to use the existing style of just
doing `stages.test`. Adding `stages/__init__.py` feels wrong and
the desire is to have the stages tests close to the stages so this
seems the least invasive way.

.github/workflows/test.yml

@@ -19,6 +19,7 @@ jobs:
         - "test.run.test_noop"
         - "test.run.test_sources"
         - "test.run.test_stages"
+        - "-k stages/test"
         environment:
         - "py36"
         - "py39"