Use safe conversion instead of unsafe constructor in issue-23433 test

Author: SimonSapin

src/test/run-pass/issue-23433.rs

@@ -16,7 +16,7 @@ use std::ptr::Unique;
 
 fn main() {
     let mut a = [0u8; 5];
-    let b: Option<Unique<[u8]>> = unsafe { Some(Unique::new_unchecked(&mut a)) };
+    let b: Option<Unique<[u8]>> = Some(Unique::from(&mut a));
     match b {
         Some(_) => println!("Got `Some`"),
         None => panic!("Unexpected `None`"),