Fix proc macro panic message in tests

test_suite/tests/compile-fail/enum-representation/internal-tuple-variant.rs

@@ -1,7 +1,7 @@
 #[macro_use]
 extern crate serde_derive;
 
-#[derive(Serialize)] //~ ERROR: custom derive attribute panicked
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
 #[serde(tag = "type")] //~^ HELP: #[serde(tag = "...")] cannot be used with tuple variants
 enum E {
     Tuple(u8, u8),