Check existing tests to only verify const auto traits parsing

Author: lapla-cogito

tests/ui/traits/const-traits/parse-const-unsafe-trait.rs

@@ -1,12 +1,14 @@
 // Test that `const unsafe trait` and `const unsafe auto trait` works.
 
+//@ compile-flags: -Zparse-crate-root-only
 //@ check-pass
 
 #![feature(const_trait_impl)]
 #![feature(auto_traits)]
 
 pub const unsafe trait Owo {}
 const unsafe trait OwO {}
+
 pub const unsafe auto trait UwU {}
 const unsafe auto trait Uwu {}