Place compile-fail expected errors on their own line

test_suite/tests/compile-fail/conflict/flatten-tuple-struct.rs

@@ -9,7 +9,8 @@
 #[macro_use]
 extern crate serde_derive;
 
-#[derive(Serialize)] //~ ERROR: 12:10: 12:19: #[serde(flatten)] cannot be used on tuple structs
+#[derive(Serialize)]
 struct Foo(u32, #[serde(flatten)] HashMap<String, String>);
+//~^^ ERROR: #[serde(flatten)] cannot be used on tuple structs
 
 fn main() {}