Mock SSL credentials for MTLS test cases

Author: lidizheng

tests/system/conftest.py

@@ -127,10 +127,22 @@ def identity():
 
 @pytest.fixture
 def async_identity():
-    transport = IdentityAsyncClient.get_transport_class('grpc_asyncio')(
-        channel=aio.insecure_channel('localhost:7469'),
-    )
-    return IdentityAsyncClient(transport=transport)
+    if use_mtls:
+        with mock.patch("grpc.ssl_channel_credentials", autospec=True) as mock_ssl_cred:
+            mock_ssl_cred.return_value = ssl_credentials
+            client = IdentityAsyncClient(
+                credentials=credentials.AnonymousCredentials(),
+                client_options=client_options,
+            )
+            mock_ssl_cred.assert_called_once_with(
+                certificate_chain=cert, private_key=key
+            )
+            return client
+    else:
+        transport = IdentityAsyncClient.get_transport_class('grpc_asyncio')(
+            channel=aio.insecure_channel('localhost:7469'),
+        )
+        return IdentityAsyncClient(transport=transport)
 
 
 @pytest.fixture