Short answer: don’t.
Testing repositories are mainly used by Arch developers as a play zone, to test new packages. You’ll be prone to system failures and bugs if you use it, and you’ll experience no real benefits. Packages are moved from testing really fast, so it’s not a big deal.
Some people activate them to install the latest version of Gnome, and so on… but as these packages are moved fast to the “normal” repositories, you just have to wait to get them, and it’s normally a matter of weeks (with big DE releases).
As Antergos is rolling release, you’ll find updated versions of all your programs (as you would find in a Debian testing) without the need of activating testing repos.