Import macros exclusively through serde_derive in test suite

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

test_suite/no_std/src/main.rs

@@ -19,7 +19,7 @@ fn panic(_info: &core::panic::PanicInfo) -> ! {
 
 //////////////////////////////////////////////////////////////////////////////
 
-use serde::{Deserialize, Serialize};
+use serde_derive::{Deserialize, Serialize};
 
 #[derive(Serialize, Deserialize)]
 struct Unit;