only check stderr on nightly

Author: KodrAus

tests/ui.rs

@@ -6,6 +6,7 @@ fn ui_compile_pass() {
  t.pass("tests/compile-pass/*.rs");
 }
 
+#[rustversion::nightly]
 #[test]
 fn ui_compile_fail() {
  let t = TestCases::new();