Fix tests after grpcio update (#5333)

* Fix tests after grpcio update
* add test for close stub for coverage

Author: crwilcox

google/api_core/grpc_helpers.py

@@ -358,3 +358,7 @@ def subscribe(self, callback, try_to_connect=False):
     def unsubscribe(self, callback):
         """grpc.Channel.unsubscribe implementation."""
         pass
+
+    def close(self):
+        """grpc.Channel.close implementation."""
+        pass

tests/unit/test_grpc_helpers.py

@@ -375,3 +375,7 @@ def test_subscribe_unsubscribe(self):
         channel = grpc_helpers.ChannelStub()
         assert channel.subscribe(None) is None
         assert channel.unsubscribe(None) is None
+
+    def test_close(self):
+        channel = grpc_helpers.ChannelStub()
+        assert channel.close() is None