Fix tests

Author: cofibrant

src/libcore/mem/mod.rs

@@ -1010,6 +1010,7 @@ pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
 ///
 /// ```
 /// # #![feature(never_type)]
+/// # #![feature(variant_count)]
 ///
 /// use std::mem;
 ///