Fix stable feature names in tests

Author: dtolnay

tests/ui/missing_const_for_fn/auxiliary/helper.rs

@@ -1,8 +1,8 @@
 // This file provides a const function that is unstably const forever.
 
 #![feature(staged_api)]
-#![stable(feature = "1", since = "1.0.0")]
+#![stable(feature = "clippytest", since = "1.0.0")]
 
-#[stable(feature = "1", since = "1.0.0")]
+#[stable(feature = "clippytest", since = "1.0.0")]
 #[rustc_const_unstable(feature = "foo", issue = "none")]
 pub const fn unstably_const_fn() {}