make(1): add test for the .for directive

For a long time, I had assumed that the iteration variables of a .for
loop are just normal global variables.  This assumption was wrong but
didn't have any consequences.

The iteration variables of a .for loop can just be accessed like global
variables, therefore it is not obvious that they are implemented in a
completely different way.

There are some edge cases in conditions used inside .for loops, in which
the iteration variables cannot be used like normal variables.  An
example is brought up in https://gnats.netbsd.org/47888, which observes
that the defined() and empty() functions in conditions only work with
variables but ignore the iteration "variables", simply because these are
not variables but only expressions.

(rillig)