test reduce aliasing arguments

[haarg]

No description provided.

[leonerd]

I don't want to assert this in the tests because the documentation never states it. What it says is:

Reduces @list by calling "BLOCK" in a scalar context multiple times, setting $a and $b each time. The first call will be with $a and $b set to the first two elements of the list, subsequent calls will be done by setting $a to the result of the previous call and $b to the next element in the list.

It just uses the phrase "sets $b to ...", doesn't mention aliasing. The current implementation happens to alias but I don't want to guarantee that in future, as that's a fragile implementation detail that might change.