xDS interop: log the subTest start and beginning (#30517)

To improve debugging of the tests with steps that look similar, f.e. failover.
Makes the end of one subtest, and the beginning of the next one much clearer.

Note: URL map test suite does not use subtests, so I didn't add the logging there.

tools/run_tests/xds_k8s_test_driver/framework/xds_k8s_testcase.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import abc
+import contextlib
 import datetime
 import enum
 import hashlib
@@ -169,6 +170,14 @@ class XdsKubernetesBaseTestCase(absltest.TestCase):
         cls.secondary_k8s_api_manager.close()
         cls.gcp_api_manager.close()
 
+    @contextlib.contextmanager
+    def subTest(self, msg, **params):  # noqa pylint: disable=signature-differs
+        logger.info('--- Starting subTest %s.%s ---', self.id(), msg)
+        try:
+            yield super().subTest(msg, **params)
+        finally:
+            logger.info('--- Finished subTest %s.%s ---', self.id(), msg)
+
     def setupTrafficDirectorGrpc(self):
         self.td.setup_for_grpc(self.server_xds_host,
                                self.server_xds_port,