I had testimony in my character sheet but dropped it cos I never had reason to use it. What’s it for? All it seems to do is… Let you name someone? In a way that shows up to insight? I guess it is to warn potential future citizens of (ic!) red flags about a person?
Also, don’t scoop names do the same thing?
This is not a part of the game I’ve poked much, and I suspect you’ll get better answers, but:
Testimony is like a tattoo on a soul.
It can be a bad thing, it can be a good thing. It’s a lasting and permanent badge for someone.
Apart from being a marker that the recipient has done deeds that are worthy of note and rememberance, as judged by a priest of the Way, it’s also a hell of an ID card.
I play in Dawn, our words are “great deeds are eternal”. So the prospect of having your great deeds noted on your soul sounds a fine idea to us.
Skop names, as I understand them, are (a) almost entirely a Wintermark thing, (b) Are worn as part of a given name of one life, not marked on a soul.
There’s some similarities, in that they are both bestowed by those who judge great deeds, as marks of shame or glory (or heroism). But they aren’t the same.
This would make an interesting IC quest for you: determine the utility of Testimony 
Highguard certainly believe that a testimony is a reward for a great deed, so that future generations will know your virtue. And they do treat it as a solemn ceremony and needing careful thought as to the words chosen.
I’ve seen it mostly as a positive thing. My wife is thinking about taking testimony specifically so she can give one to someone as a token of gratitude.
Scop/hero names are “culture locked,” only associated with Wintermark and there’s been some IC conversations about it and assorted bits
Testimony is an RP heavy “fun button” that can be used as a way of rewarding someone, shaming someone, or just noting the cool thing they did that you got to observe
My last character got the testimony “Physick’s Assurance” due to some personal plot bits that got observed
Current character got the very silly testimony of “Meddlesome Bat,” after the ??? Time he nagged a character to look after themself and got others in on ensuring that said character looked after themself
Partner’s character has recently been given the testimony “Urizen’s Artisan”
I know someone who, after an absurd sequence of events, got the very ridiculous testimony “Doctor Sausage”
There’s someone else I know who has “Skillful poisoner” as one of their testiony
In all cases, they’re little story beats and points in time associated with things done.
Out of curiousity, since it’s a marker on the soul, would testimony statements & titles carry over to a reincarnated soul to be read in the (potentially distant) future?
It’s something that’s theorised IC to occur (as some past life visions end up with the visionary having marker from previous life on soul)…but it’s not a solid y/no and lots of IC theories abound