xds-k8s: Fix regression with returning multiple test servers (#26786)

Introduced in #26759

tools/run_tests/xds_k8s_test_driver/framework/test_app/server_app.py

@@ -20,7 +20,7 @@ modules.
 import functools
 import logging
 import threading
-from typing import Iterator, Optional
+from typing import Iterator, List, Optional
 
 from framework.infrastructure import gcp
 from framework.infrastructure import k8s
@@ -198,7 +198,7 @@ class KubernetesServerRunner(base_runner.KubernetesBaseRunner):
             maintenance_port=None,
             secure_mode=False,
             server_id=None,
-            replica_count=1) -> Iterator[XdsTestServer]:
+            replica_count=1) -> List[XdsTestServer]:
         # Implementation detail: in secure mode, maintenance ("backchannel")
         # port must be different from the test port so communication with
         # maintenance services can be reached independently from the security

tools/run_tests/xds_k8s_test_driver/framework/xds_k8s_testcase.py

@@ -317,7 +317,7 @@ class RegularXdsKubernetesTestCase(XdsKubernetesTestCase):
             replica_count=replica_count,
             test_port=self.server_port,
             maintenance_port=self.server_maintenance_port,
-            **kwargs)
+            **kwargs)[0]
         test_server.set_xds_address(self.server_xds_host, self.server_xds_port)
         return test_server
 
@@ -397,7 +397,7 @@ class SecurityXdsKubernetesTestCase(XdsKubernetesTestCase):
             test_port=self.server_port,
             maintenance_port=self.server_maintenance_port,
             secure_mode=True,
-            **kwargs)
+            **kwargs)[0]
         test_server.set_xds_address(self.server_xds_host, self.server_xds_port)
         return test_server