Place compile-fail expected errors on their own line

test_suite/tests/compile-fail/with-variant/skip_ser_whole_variant.rs

@@ -9,10 +9,11 @@
 #[macro_use]
 extern crate serde_derive;
 
-#[derive(Serialize)] //~ ERROR: 12:10: 12:19: variant `Unit` cannot have both #[serde(serialize_with)] and #[serde(skip_serializing)]
+#[derive(Serialize)]
 enum Enum {
     #[serde(serialize_with = "serialize_some_unit_variant")]
     #[serde(skip_serializing)]
+    //~^^^^ ERROR: variant `Unit` cannot have both #[serde(serialize_with)] and #[serde(skip_serializing)]
     Unit,
 }