Nil

I mean, I assume that the test would be part of it- but how could Nightblood have fed on Szeth? He had lost the ability to draw in stormlight at that point and was newly not-dead. If wielding the sword is "one of the secrets" as to how Vasher is immune to it, couldn't death be another? He states that would make you immune. We know Szeth couldn't safely wield the sword yet- maybe that means he's partially bonded to the sword, but still has more to do?

So nightblood makes anyone who it views as not evil nauseated, and anyone who is evil is compelled to kill themselves; with a few notable exceptions. The only people who have no reaction to nightblood, either nausea or temptation, have been Vasher, Nale (assumedly), and Szeth. I believe Denth was also unaffected, to the point of being able to throw the sword away. (We didn't see too much of the other sky breakers in edgedancer around the sword, but I'm assuming it had the regular effect on them) The one thing all those people have in common the others didn't is that they have died, and returned through some form of investiture. I would guess that that means the ability to wield nightblood has something to do with death and/or the cognitive realm. But what im really getting at here is at some point Kelsier should totally wield nightblood.