add missing feature flag

Author: RalfJung

src/test/ui/consts/cast-discriminant-zst-enum.rs

@@ -1,5 +1,6 @@
 // run-pass
 // Test a ZST enum whose dicriminant is ~0i128. This caused an ICE when casting to a i32.
+#![feature(test)]
 use std::hint::black_box;
 
 #[derive(Copy, Clone)]