I do use the Tools tag. I imagine people struggle with how to test their games as they make them so I was thinking the Testing tag could have some broad appeal.
I'm on the fence about a TDD tag. I don't want my very own tag, hehe. There are two frameworks for Godot that I know of (GUT and WAT). I was thinking that the tag might increase visibility and get used more if it existed. TDD is a pretty well know design pattern but isn't widely used in game development...yet.