The force applied to the wings in flex testing is 500N (roughly 50kg).
It's pretty simple though, as soon as Ferrari really gets accused of cheating, they'll just return the favor (and rightly so) and we'll have another "multiple-team-wing-upgrading-session", like the one we saw after the first (two?) race(s) of this season.
It's indeed the pott-kettle-black thing going on here.
By changing the used force in testing, to one that's a bit more realistic, they could probably narrow down the opportunities to "cheat legally", at least in this particular area.