The idea is that the taster will taste the food, not die and pass it on.
a. Why not test it on the dog?
b. What if it's a slow poison?
While waiting for a more direct answer to your question, you might appreciate (as did I) u/jetpacksforall's answer to "Are there many cases of a 'food taster' actually consuming poison and dying on the spot?":
<Answer deleted for compliance>