Porting `test` crate to a custom target?

I have a custom target file defined, and I'd like to run tests on it. Obviously, I'll need more than just libcore support, but I'm not sure exactly how much more.

When I run cargo test --target x86_64-unknown-xen, I get the following error message:

src/main.rs:1:1: 1:1 error: can't find crate for `test` [E0463]

This suggests that cargo test depends on some crate named test, and that I should port it to my target if I want to run cargo test. Such a crate does not exist on crates.io, nor is it documented on doc.rust-lang.org.