Add FakeStreamUnaryCall to ease mocking

Author: lidizheng

google/api_core/grpc_helpers_async.py

@@ -250,3 +250,24 @@ def __init__(self, response=object()):
     def __await__(self):
         response = yield from self._future.__await__()
         return response
+
+
+class FakeStreamUnaryCall(_WrappedStreamUnaryCall):
+    """Fake implementation for stream-unary RPCs.
+
+    It is a dummy object for response message. Supply the intended response
+    upon the initialization, and the coroutine will return the exact response
+    message.
+    """
+
+    def __init__(self, response=object()):
+        self.response = response
+        self._future = asyncio.get_event_loop().create_future()
+        self._future.set_result(self.response)
+
+    def __await__(self):
+        response = yield from self._future.__await__()
+        return response
+
+    async def wait_for_connection(self):
+        pass