Issue 1069/document explicit loop for tests (#1109)

* Added args example in explicit loop testing docs

Before only loop was being passed to the constructor example.
If the service required args, the example would fail. With the new example,
any args and kwargs received from the calling service will be passed
correctly to the tested service

* Modified writing testable service section for consistency

docs/testing.rst

@@ -356,7 +356,8 @@ in its signature. An example would be the following::
   class TestAcceptance:
 
       async def test_get(self, test_client, loop):
-          with patch("main.AioESService", MagicMock(return_value=AioESService(loop=loop))):
+          with patch("main.AioESService", MagicMock(
+                  side_effect=lambda *args, **kwargs: AioESService(*args, **kwargs, loop=loop))):
               client = await test_client(create_app)
               resp = await client.get("/")
               assert resp.status == 200