I think she has the ability to learn, and like most sul'dam with this capability, appears to be very close to implicitly channeling. I think she'll get to the point where she starts channeling without her conscious control - she is young enough and ignorant enough, though, to think that this won't happen, encounter the channeling sickness because she is blocking herself against it, and then need to learn.

I imagine that Tuon wasn't tested after she had already undergone the test, and I don't think anyone would have thought to test someone of her rank more than once.

That would be a very convenient way to make her re-think her position on the humanity of channelers.