tweak const-valid test

Author: RalfJung

src/test/ui/consts/const-eval/valid-const.rs

@@ -1,7 +1,8 @@
-// build-pass (FIXME(62277): could be check-pass?)
+// check-pass
 
 // Some constants that *are* valid
 #![feature(const_transmute)]
+#![deny(const_err)]
 
 use std::mem;
 use std::ptr::NonNull;