You don’t end the test, but you don’t point out the tool either.
If someone is hopelessly stuck, then obviously for summative purposes you score that session as “Unable to complete task (without help)” and include it in the No Joy category for statistical purposes. As long as you have consistent rules for judging when the user cannot continue on his/her own, your quantitative data will be perfectly valid, and you can continue the session and collect more data to help inform the design.
The reason for not pointing out the tool is, ironically, to collect more data. You know the user couldn’t find the tool, but you probably don’t know why. In general, an inability to find something on a page may be because:
Users looked at it, but it didn’t recognize the label/icon.
Users looked towards it but didn’t see it because it was lost in clutter.
Users were looking for it somewhere other than where you put it.
Users were looking for something entirely different than what you used.
Each of these reasons has very different design responses. Eye-tracking data can help narrow the possible reasons, but interview data is also often necessary.
So don’t show the user the tool. First, ask questions to diagnose the problem:
What are you trying to do? (I need to zoom in)
What are you looking for to zoom? (A sliding thingy, like in Google maps).
Okay, that’s a good way of doing it, but we’re trying out a different method. What label or icon would Zoom have? (I don’t know. Usually it’s a magnifying glass)
(pause)
Where are you looking for it? (Right here at the top.)
What do you see there? (A printer for printing, the Save icon, a push-pin to mark a point.)
Oh. The push-pin is supposed to be a magnifying glass. We’ll work on that.
There. Now you know two ways to improve the design (use a slider if feasible, re-work the magnifying glass image), and four ways not to improve it (changing from a magnifying glass to some other object, making the control bigger or bolder, moving it somewhere else).
This is the general rule for usability testing. For each problem the user encounters, avoid giving the solution. But don't just give up. Instead, ask questions to gather data and, in the process, guide the user progressively closer to the solution. And then continue with the usability test.