Fix declare_ser_tests to work on old rustc

testing/tests/macros.rs

@@ -5,7 +5,7 @@ macro_rules! declare_ser_tests {
             #[test]
             fn $name() {
                 $(
-                    self::serde_test::assert_ser_tokens(&$value, $tokens);
+                    assert_ser_tokens(&$value, $tokens);
                 )+
             }
         )+

testing/tests/test_ser.rs

@@ -7,6 +7,7 @@ extern crate serde_test;
 use self::serde_test::{
     Error,
     Token,
+    assert_ser_tokens,
     assert_ser_tokens_error,
 };