Add doctest to the proc-macro derive-test

Author: keeperofdakeys

src/test/run-pass-fulldeps/proc-macro/derive-test.rs

@@ -17,6 +17,9 @@ extern crate proc_macro;
 
 use proc_macro::TokenStream;
 
+// ```
+// assert!(true);
+// ```
 #[proc_macro_derive(Foo)]
 pub fn derive_foo(_input: TokenStream) -> TokenStream {
     "".parse().unwrap()