validate behaviour of shadow variables

[Licenser]

In many places we just reserve but do not register variables, meaning they're going to be set in the same allocation spot in our local array. This, in theory, could cause problems. Sadly in practice, we've not yet been able to find a script that does turn this theoretical problem into a real problem. We should investigate this. This also touches on modular subqueries as shadow variables are used in the arg replacement (it uses an expression path for this).