Convert rustc_serialize integration tests to unit tests.

Because (a) the vast majority of compiler tests are unit tests, and (b)
this works better with `unused_crate_dependencies`.

compiler/rustc_serialize/src/lib.rs

@@ -3,8 +3,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::internal)]
-// FIXME
-#![allow(unused_crate_dependencies)]
 #![cfg_attr(test, feature(test))]
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
@@ -18,6 +16,10 @@
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end
 
+// Allows macros to refer to this crate as `::rustc_serialize`.
+#[cfg(test)]
+extern crate self as rustc_serialize;
+
 pub use self::serialize::{Decodable, Decoder, Encodable, Encoder};
 
 mod serialize;