I have a question about Inception (Spoilers)

How come when Mal is threatening to jump out the window and trying to convince Leo that they’re still living in the Matrix, she pleads with him to take a "leap of faith"? Isn’t her spinning top the ultimate link to reality? Why doesn’t she just spin it for him? If it stops, they’re in reality. If it doesn’t, they should jump.

And given the last scene: those who subscribe to the "the top will keep spinning" theory, are we to assume Leo has never done this test before?

This seems to me to be a very serious plothole.