Keeping two separate folders, as in, having the live game folder backed up at an external location (i.e. desktop) then reinstalling the game with the testlive beta branch at its normal installation folder. Once you want to go back to the live game, just swap the folders.
It’s what I’ve personally used for a few months now, to make sure I don’t stumble into this kind of issue. A bit of a clunky workaround, but works. 
Edit: Just realised, I forgot to also mention! When swapping the folders, remember to also set the game branch to its correct setting as well, otherwise Steam will override the new files once it checks game integrity!