A Test of Will is a card which I can never leave out of a deck that uses . The moment I resolve on that sphere I put in 3 copies of Test of Will. And that's one of the reasons why I really dislike the card. To be clear before I start complaining about it, A Test of Will is possibly the best card in the entire game for pure power. One resource cancels a When Revealed effect from the encounter deck, no other conditions, that effect just straight up doesn't happen. The effect is demonstrably worth paying more than that 1 resource for given how often people will pay extra by playing Dwarven Tomb to retrieve it, or Map of Earnil to retrieve Dwarven Tomb to retrieve Test of Will. And the fact it's so easy makes it to my mind the biggest example in the game of an auto-include card by a very wide margin. It's just so universal and so easy that you can't justify omitting it, and that makes for boring deckbuilding.
It also raises issues with regards to encounter deck design because really quests should be designed on the assumption that the effects in the encounter deck will actually happen. Now I want to be clear - I'm not necessarily advocating for there to be no cancellation of When Revealed effects in the game at all, it's just that Test of Will is a bit too easy, being only 1-cost and no other restrictions on it.
Now earlier in the life of the game there were some treacheries which had ridiculously brutal effects - the infamous "cancel-or-die" which is another thing I really dislike in this game. But my logic here is that those cards were created under the (in my opinion flawed) mindset of "It's fine, because they can cancel it with Test of Will." Following that logic through, if it was either not possible or at least not as easy to cancel such effects, then "cancel-or-die" effects wouldn't be a thing (Since one would at least hope the designers would stop short of effects that were effectively just "die"). Therefore if A Test of Will didn't exist (or was more expensive in some way), then Sleeping Sentry (and similarly ridiculous cards) wouldn't exist, and I feel like I might prefer that version of the game. I feel like it'd be better balanced that way.
Regardless of feelings on how it would have impacted the game balance, I think it's pretty clear that a card of such incredible power as Test of Will absolutely would not be released nowadays. I mean, imagine if Test of Will wasn't in the Core Set, so our only cancellation was Eleanor (or perhaps go even further and imagine Eleanor didn't exist either). Imagine that was the case, most or all decks in the game had no means of cancelling When Revealed effects for 5 years, and then Test of Will was released in the latest adventure pack. People would go crazy about how overpowered it was. The only reason it tends to get a pass is because it was in the Core Set so everyone's used to it existing at this point and they don't really think about it. Or if they do think about it, then it probably saved so many games for them that they naturally feel well-disposed towards it. If a card like Test of Will were released now, it would be more expensive or more limited. It wouldn't just be a 1-cost event.
And then again, I'm also not in any way advocating for the removal of Test of Will from the card pool, and I will continue to include 3 copies in every deck I build forever, most likely. If I wanted to remove it, I'd need to somehow acquire a time machine so I could go back and stop Nate French putting it into the Core Set in the first place, and then we'd see the ripple effects impacting the design of treacheries. Since it was in the Core Set, it's way too late to change it now because the game has been designed to some extent to allow for it existing. That said, I do sometimes find it refreshing when I play more recent quests (which don't have the "cancel-or-die" problem) to use decks without , so when a treachery is revealed there's no lamenting my failure to draw cancellation, no debate over whether this is worth cancelling or if I should save it for something worse later on, the effect just happens and I have to deal with it happening.
In conclusion/summary, Test of Will is a ridiculously powerful card which you should basically never leave out of any deck that can play it. Perhaps we may speculate that things might have been better had it never been created, but even if so it's no use lamenting a past which we cannot change. As Gandalf says, "All we have to decide is what to do with the time that is given us." And in this case, that involves simply accepting that this is one of the most, if not straight up the most powerful card in the entire game, and thus that 3 slots in every deck are automatically spoken for regardless of anything else.