Ensure a function call throws a warning?

[dourouc05]

There is already @fact_throws to check whether a function call gives an error or an exception, but no @fact_warns for warnings.

I think the syntax should be very similar to that of @fact, as a warning does not interrupt the function execution: it still returns a value, but prints something on the output.

(And I have no idea about how to implement such a thing. May be with STDERR redirection? I've yet to make it work…)