Remove some warnings and make tests pass.

src/libcore/vec.rs

@@ -2193,13 +2193,6 @@ mod tests {
         assert addr == addr_imm;
     }
 
-    #[test]
-    fn test_unshift() {
-        let mut x = [1, 2, 3];
-        unshift(x, 0);
-        assert x == [0, 1, 2, 3];
-    }
-
     #[test]
     fn test_capacity() {
         let mut v = [0u64];