Move serde_test to dev-dependencies.

src/lib.rs

@@ -1,5 +1,4 @@
 extern crate serde;
-extern crate serde_test;
 extern crate unreachable;
 
 use std::fmt;
@@ -11,7 +10,6 @@ use std::collections::HashMap;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use serde::ser::SerializeSeq;
 use serde::de::{SeqAccess, Visitor};
-use serde_test::{assert_tokens, Token};
 
 mod hasher;
 
@@ -415,6 +413,10 @@ impl<'de> Deserialize<'de> for BitSet {
 
 #[cfg(test)]
 mod tests {
+    extern crate serde_test;
+
+    use self::serde_test::{assert_tokens, Token};
+
     use super::*;
 
     #[test]