What happens if my Goblin Test Pilot ends up targeting my opponent’s Kopala?
When you activate Goblin Test Pilot's ability, you choose from among legal targets for its ability during the activating process. If that target happens to be Kopala or anothehr Merfolk controlled by that player, then an additional cost of 2 is imposed on the costs of the ability. You're then free to pay it (and in fact must if you have 2+ mana in your mana pool), but if you don't have the mana in your mana pool, you're not forced to generate it.
Since you then can't pay the costs to activate the ability, the entire activation is undone. You're then free to try activating the Test Pilot again. You're free to do this until the Test Pilot targets a non-Merfolk target if you like, but if you go around abusing this interaction, expect to get some sour looks from the other players in your game.
Note that this only works because Kopala actually imposes an additional cost on the ability. Something like Ward is just a triggered ability and doesn't allow these sorts of shenanigans.
