[tests] explicitly disable border agent in test_publish_meshcop_service (#11713)

The `test_publish_meshcop_service` is updated to explicitly disable
the border agent before factory resetting `br`. This makes the
test more reliable by ensuring the MeshCoP service is unpublished.

tests/scripts/thread-cert/border_router/test_publish_meshcop_service.py

@@ -179,6 +179,8 @@ class PublishMeshCopService(thread_cert.TestCase):
         self.check_meshcop_service(br1, host)
         self.check_meshcop_service(br2, host)
 
+        br1.disable_border_agent()
+        self.simulator.go(5)
         br1.factory_reset()
 
         dataset = {