Mock out stackdriver client in test case (#170)

Author: wkiser

tests/unit/trace/ext/flask/test_flask_middleware.py

@@ -119,7 +119,13 @@ def test_init_app_config_stackdriver_exporter(self):
             },
         }
 
-        middleware = flask_middleware.FlaskMiddleware()
+        class StackdriverExporter(object):
+            def __init__(self, *args, **kwargs):
+                pass
+
+        middleware = flask_middleware.FlaskMiddleware(
+            exporter=StackdriverExporter
+        )
         middleware.init_app(app)
 
         self.assertIs(middleware.app, app)