Add dns_helper_unit_test in TEST_MAPPING

Enable test in postsubmit.

According to go/test-mapping-slo-guide, the test needs to meet
postsubmit SLO before adding it to presubmit.

Bug: 288340533
Test: run presubmit
Change-Id: Id5fae55fa0cb7c7b15e183bbea66c7a139360d95
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 46308af..520124d 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -131,6 +131,9 @@
       "keywords": ["netd-device-kernel-4.9", "netd-device-kernel-4.14"]
     },
     {
+      "name": "dns_helper_unit_test"
+    },
+    {
       "name": "traffic_controller_unit_test",
       "keywords": ["netd-device-kernel-4.9", "netd-device-kernel-4.14"]
     },