Import macros exclusively through serde_derive in test suite

This makes it easier to execute tests against the precompiled serde_derive.

test_suite/tests/test_macros.rs

@@ -8,7 +8,7 @@
 
 mod bytes;
 
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 use serde_test::{
     assert_de_tokens, assert_de_tokens_error, assert_ser_tokens, assert_tokens, Token,
 };