Avoid calling the app fixture directly

This behaviour is deprecated and is to be removed in pytest 4.

routemaster/conftest.py

@@ -264,10 +264,24 @@ def json(self):
         return json.loads(self.data)
 
 
+def get_test_app(**kwargs):
+    """Instantiate an app with testing parameters."""
+    return TestApp(Config(
+        state_machines=kwargs.get('state_machines', TEST_STATE_MACHINES),
+        database=kwargs.get('database', TEST_DATABASE_CONFIG),
+        logging_plugins=kwargs.get('logging_plugins', [
+            LoggingPluginConfig(
+                dotted_path='routemaster.logging:PythonLogger',
+                kwargs={'log_level': 'DEBUG'},
+            ),
+        ]),
+    ))
+
+
 @pytest.fixture()
 def client(custom_app=None):
     """Create a werkzeug test client."""
-    _app = app() if custom_app is None else custom_app
+    _app = get_test_app() if custom_app is None else custom_app
     server.config.app = _app
     _app.logger.init_flask(server)
     return Client(wrap_application(_app, server), TestClientResponse)
@@ -276,22 +290,13 @@ def client(custom_app=None):
 @pytest.fixture()
 def app(**kwargs):
     """Create an `App` config object for testing."""
-    return TestApp(Config(
-        state_machines=kwargs.get('state_machines', TEST_STATE_MACHINES),
-        database=kwargs.get('database', TEST_DATABASE_CONFIG),
-        logging_plugins=kwargs.get('logging_plugins', [
-            LoggingPluginConfig(
-                dotted_path='routemaster.logging:PythonLogger',
-                kwargs={'log_level': 'DEBUG'},
-            ),
-        ]),
-    ))
+    return get_test_app(**kwargs)
 
 
 @pytest.fixture()
 def custom_app():
-    """Return the app config fixture directly so that we can modify config."""
-    return app
+    """Return the test app generator so that we can pass in custom config."""
+    return get_test_app
 
 
 @pytest.fixture()