runner: make sure SetupState._finalizers is always set for a node in the stack
runner: make sure SetupState._finalizers is always set for a node in the stack

This makes the stack <-> _finalizers correspondence clearer.

nicoddemus/pytest,pytest-dev/pytest,RonnyPfannschmidt/pytest,nicoddemus/pytest,markshao/pytest,Akasurde/pytest