So by a "trial" you mean the process of "trying" something, as in to examine and make a decision regarding this thing.
At law, the term would be "trier", or specifically "trier of fact", so you're right that it would be correct usage, but very formal. A trial imples an examination and a decision or judgement, so "decider" or "judge" would work in general ("I'll be the judge of whether this is good or not", "I'll decide that for myself"), but not at law, and referring to someone else as a "judge" is not done flippantly because the term implies an official position as the decision-maker (which could also work for you).
An archaic but possibly useful term is "querent", which is simply "one who queries/questions". The term is most often used to refer to a person who seeks advice from a medium, such as an oracle, tarot reader or fortune teller. But, the general idea is that the person doing the trying is questioning something about the tried, and desires an answer, so depending on your context it may work.
Almost always, the person doing the trying can and should be referred to by some more specific term depending on the circumstances of the trial. At law, the "trier of fact" is the jury if there is one, the judge or judicial panel if not. An everyday person who is trying something is simply a "person"; if in the context of trying a product before buying, they're a "customer". If trying to reach a goal, especially in a competition, the person is a "competitor".