Fix tests after grpcio update (#5333)

* Fix tests after grpcio update
* add test for close stub for coverage
diff --git a/google/api_core/grpc_helpers.py b/google/api_core/grpc_helpers.py
index 329971e..6a2f052 100644
--- a/google/api_core/grpc_helpers.py
+++ b/google/api_core/grpc_helpers.py
@@ -358,3 +358,7 @@
     def unsubscribe(self, callback):
         """grpc.Channel.unsubscribe implementation."""
         pass
+
+    def close(self):
+        """grpc.Channel.close implementation."""
+        pass
diff --git a/tests/unit/test_grpc_helpers.py b/tests/unit/test_grpc_helpers.py
index a5bcced..e5e4311 100644
--- a/tests/unit/test_grpc_helpers.py
+++ b/tests/unit/test_grpc_helpers.py
@@ -375,3 +375,7 @@
         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