I agree that a practice mode would be a cool feature, but for now, you can achieve the same result with the console. Give yourself any item, talent, create any enemy, test whatever you want. On the discord we have an explanation of the console and lists of commands that can do these things.
Viewing post in Suggestions : Practice Range /and/ Horde Mode
There is a link in the game description, does it work for you?
If not, here is another: https://discord.gg/Fw4SHhpfpw