I wouldn't "bet" on anything in the Ayra story.
The followings are my guesses:
It's my guess is that wither or not there is something going on with faces is about 50/50 (there is a 50% chance that that really was the waif, really stabbing real Ayra in the gut, and I'm going to run with that contingency, rather then the other likely one that "Ayra" in the episode was in fact the no named man testing the Waif)
But the lead up to the scene was too open to tell
On the surface:
we saw Ayra go to sleep with her reclaimed sword two episodes ago, I was expecting a night time encounter with the waif
But then on last weeks episode she was out and about without her sword and her personality was decidedly different then normal too.
My suspicions are it was genuinely her that got attacked, but that she had planned to "be wounded, but get away", She's been beaten in sparring matches by the Waif for about 600 seasons so, if I know my favorite character (competitive, studious, always thinking) She concocted a plan to kill the Waif, as a sort of forfeit/payment to the no named man, and she's doing it by appealing to the Waif's predatory nature and jealously of her.
The Waif probably wants Ayra to "suffer" and doesn't mind ignoring the no named man's request, which is how Ayra knew she would not kill her outright, instead she will wound her then hunt her down and then kill her.
Ayra's blood splatter hand prints that she "accidentally" left on the walls as she escaped will no doubt make it easier for the Waif to follow her to wherever Ayra is leading them both.
Its my guess that she is leading them somewhere where she will confront the waif (possibly with an advantage, perhaps somewhere with poor visibility) and kill her in a way the no name man will easily find, and then she will successfully leave having fullfilled the no name man's words that "one way or another a face will be added to the halls"