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