Use updated local declaration syntax for xfail'd test

This patch is trivial, but it comes with a question. Does Mozilla need to own the copyright on code submitted to Rust?

The reason I ask is that, since the last time I submitted anything to a Mozilla project, I started working at Google, and they (by default) own the copyright on code that I write (even in my spare time). There's a process to assign copyright to another entity, and it should be a formality for something like this, but I'd still have to go through it if that's a requirement for Rust.

Anyway, I'm submitting this incredibly trivial thing because, if I have to go through that process for the first time, I'd like it to be for something that's trivial, so I can see how much of a hassle it is (if any) without having invested much time up front.

I didn't see anything about copyright in the Mozilla contributor's agreement, but I could have easily missed something somewhere else.