Where can a 14-year-old get a job as a game tester

For the majority of Publishers they won't look at you until you are 18. You need to sign what is called an NDA(Non-Disclosure Agreement) and you need to be an adult to do that.

Minor work laws prohibit anyone under 14 for sure.

You can start getting some experience and education in Video game testing earlier. See the program at PowerUpGames.com