Testing your mettle was a reasonably common combat arts term when I was growing up because as you went through your training until you had been in a combat situation, you were regarded as untested. Being untested is not actually a bad thing at the beginning of your journey, but at some point, you either have to compete if you are in a combat sport or use your art in self-defence if and when the time comes.
My first Muay Thai teacher said you were not a Muay Thai fighter until you had completed twice. Once was not enough; you had to return to the ring despite knowing how hard it was. It had to be a full-on match, not sparring or hard pad drill training. When I was going through some sniper training, the head of the course said you are unproven until you get your first two kills despite passing the course with flying colours and getting the funny little rifle badge to wear. He actually asked us to write to him and confirm when we had at least two kills because only then did he consider his work was successfully done. I later found out that some people passed the course and got their first kill but found they did not have the will to do so again. I can fully understand that.
I was trained to hunt from about four onwards, but as an adult, I had no idea how I would react when the time came; I knew there was a fair chance of being in the position to kill as I was in the military then working free-lance. I did a heap of training, the live-fire drills and simulations, putting thousands of rounds down the range, but you never really know, and even when you have been successful in a previous encounter, you never know how things will pan out the next time.
I noted above that it’s not a problem if you are untested because we all are at first, and sometimes in the case of self-protection, you have to wait to be tested. The issue comes when you move on to teach others; imagine a sniper teaching who never made a kill. They may understand stealth and trig maths and have marksmanship skills but have never performed the live task. Likewise, anyone teaching combat sports ideally would have been turning in outstanding live performance at least at some point in their career.
Regarding modem martial arts training, unless it’s a combat sport or your teacher has a side hustle bouncing bars, it’s unlikely they will be proven. I would argue it’s actually getting rarer to find a proven teacher due to how our legal system works these days. I have friends who teach and are very honest about the fact they never had to deal with actual combat. Is that a significant problem? No, as long as they are honest with their students.
For martial arts teachers who can, I would wholly recommend taking part in combat sports, like grappling or kickboxing or MMA, to road test yourself. It will help you design a training program that will prepare your students to perform well when their test comes.