Remove #[cfg(not(test))] gates in core

These gates are unnecessary now that unit tests for `core` are in a
separate package, `coretests`, instead of in the same files as the
source code. They previously prevented the two `core` versions from
conflicting with each other.

library/core/src/array/ascii.rs

@@ -1,6 +1,5 @@
 use crate::ascii;
 
-#[cfg(not(test))]
 impl<const N: usize> [u8; N] {
     /// Converts this array of bytes into an array of ASCII characters,
     /// or returns `None` if any of the characters is non-ASCII.