[Backport grpc#27373] add testing_version flag (#4783)

Author: lidizheng

test/kokoro/xds_url_map.sh

@@ -89,6 +89,7 @@ run_test() {
     --flagfile="${TEST_DRIVER_FLAGFILE}" \
     --kube_context="${KUBE_CONTEXT}" \
     --client_image="${CLIENT_IMAGE_NAME}:${GIT_COMMIT}" \
+    --testing_version=$(echo "$KOKORO_JOB_NAME" | sed -E 's|^grpc/go/([^/]+)/.*|\1|') \
     --xml_output_file="${TEST_XML_OUTPUT_DIR}/${test_name}/sponge_log.xml" \
     --flagfile="config/url-map.cfg"
   set +x