Extern crate warnings in test suite

[dtolnay]

As of rustc 1.21.0-nightly (e26688824 2017-08-27), cargo test gives the following warnings.

   Compiling serde_test_suite v0.0.0
warning: unused extern crate
  --> tests/test_identifier.rs:12:1
   |
12 | extern crate serde;
   | ^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_extern_crates)] on by default

warning: unused extern crate
  --> tests/test_ser.rs:21:1
   |
21 | extern crate serde;
   | ^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_extern_crates)] on by default

warning: unused extern crate
  --> tests/test_remote.rs:12:1
   |
12 | extern crate serde;
   | ^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_extern_crates)] on by default

[dtolnay]

Fixed by rust-lang/rust#44825.