/run C_PetBattles.ForfeitGame()Simply create a macro, place it on your bars, and then keybind it. Once you're in a battle, just hit the bind and you'll instantly forfeit the match. You won't have to click on the confirmation box or anything!
Do what you will with this macro, but personally I see it most useful for Tamer battles.
Can't get the right lineup that you want? Use the macro! Your team looks like it's not going to survive the fight? Use the macro! Change your mind about what pets you want to use? Use the macro! Realize that you've selected the wrong abilities for a pet? Use the macro! It just helps speed things up a little bit. :)
Of course, this new trick does have a downside. It makes despawning wild pets in the search for rares that much quicker. Not everyone practices this method of looking for rare quality companions, but for those that do, it will only speed up the process. This might leave very few wild pets behind for others to battle and/or capture.
So if anything, I urge user discretion with this forfeit macro. It can come in handy for personal use, but remember that using it on wild pets (which are meant for everyone, not just individuals) could impact another person's pet collecting experience.