Super-simple question: can Frozen by Fear ("When Revealed: Until the end of the round, players cannot trigger Action or Response effects.") be canceled by A Test of Will, a Response effect?

I've been playing it as no.

Yes, it can. If an effect is cancelled (as by Test of Will), it never happens. You don't apply the effect and then undo it - at the time that Test of Will is played Frozen by Fear hasn't resolved its effect and so it is still valid to play Test of Will.

*shudders*

I remember that treachery, AWFUL card especially when you use heroes that have strong response effects that your decks build upon. Elrohir and Elladan for example are rendered almost useless by it..

It can definitely be cancelled by Test of Will.