Fix proc macro panic message in tests

test_suite/tests/compile-fail/str_ref_deser.rs

@@ -1,7 +1,7 @@
 #[macro_use]
 extern crate serde_derive;
 
-#[derive(Serialize, Deserialize)] //~ ERROR: custom derive attribute panicked
+#[derive(Serialize, Deserialize)] //~ ERROR: proc-macro derive panicked
 struct Test<'a> {
     s: &'a str, //~^^ HELP: Serde does not support deserializing fields of type &str
 }