Remove unused extern crate test

Recently we removed the "unstable" feature, I missed the duplicate
`extern crate test` when doing so :(

Since we no longer have the "unstable" feature this line of code is
never compiled in.

Remove the unused ``extern crate test`, we have the correct line further
up the file `#[cfg(bench)] extern crate test;`.

Author: tcharding

src/lib.rs

@@ -85,8 +85,6 @@ extern crate bincode;
 extern crate serde_json;
 #[cfg(all(test, feature = "serde"))]
 extern crate serde_test;
-#[cfg(all(test, feature = "unstable"))]
-extern crate test;
 
 #[cfg(test)]
 #[macro_use]