It seems like the worldborder damage is not working in the latest snapshot

1. Set worldborder center

   /worldborder center ~ ~ 
   

2. Set worldborder size

   /wordborder set 10
   

   Watch mobs outside the worldborder, they won't get hurt, if there are no mobs you can teleport outside

   /tp ~ ~ ~13
   

   and then summon some

   /summon Pig