xds-k8s: Fix incorrect type hint in the baseline test (grpc#29577)

Author: sergiitk

tests/baseline_test.py

@@ -44,16 +44,14 @@ def test_traffic_director_grpc_setup(self):
         with self.subTest('4_create_forwarding_rule'):
             self.td.create_forwarding_rule(self.server_xds_port)
 
-        test_servers: _XdsTestServer
         with self.subTest('5_start_test_server'):
-            test_servers = self.startTestServers()
+            test_server: _XdsTestServer = self.startTestServers()[0]
 
         with self.subTest('6_add_server_backends_to_backend_service'):
             self.setupServerBackends()
 
-        test_client: _XdsTestClient
         with self.subTest('7_start_test_client'):
-            test_client = self.startTestClient(test_servers[0])
+            test_client: _XdsTestClient = self.startTestClient(test_server)
 
         with self.subTest('8_test_client_xds_config_exists'):
             self.assertXdsConfigExists(test_client)