Use factories and fixtures in pubsub tests

Done
- Add factories using factory-boy
- Modify fixtures and tests to use factories
- Modify tests to use fixtures and factories
- Clean up

tests/pubsub/test_dummyaccount_demo.py

@@ -30,7 +30,7 @@ async def perform_test(num_nodes, adjacency_map, action_func, assertion_func):
 
     # Create nodes
     dummy_nodes = []
-    for i in range(num_nodes):
+    for _ in range(num_nodes):
         dummy_nodes.append(await DummyAccountNode.create())
 
     # Create network