Hello,
I am a long time Debian Testing user, and would like to use the testing repo on armbian as well.
Currently however, I only get the choice between stable and unstable. apt.armbian.com also seems to only house packages for stable and unstable, not for testing.
I would think that when you are already supporting stable and unstable, testing wouldn't be much more than just flicking a switch to turn it on. However, I assume people here know better than me. I barely know what is in the armbian repos.