Had this happen to me as well, among several other tonic related issues. Only thing that got me past the infinite "taste test" loop was to use my other character (one that is not bugged), and start brewing any tonic (would recommend a cheap green one). Only go up to the point where it says "taste test" and do not go any farther. Do not taste test it. Swap back to the bugged character and finish the brewing steps by taste testing and then brewing the tonic. I had to do it twice to fix the issue. Hunter was bugged at infinite "Taste test", used Warlock to get to the taste test part, swapped to bugged Hunter, but still couldn't proceed. Swapped to Titan, got the quest to taste test, swapped to Hunter, and it fixed it. Obviously this only works if you have an alt character that can brewing potions. If you only have one character, it may be worth it to create a new one, and advance until you can craft potions with the new character. Then try the above steps. Sounds like a lot to overcome a bug, but it actually worked for me, and really didn't take much longer than 4 loading screens. It took weeks for me to figure out a fix that worked. Good luck.