Update and rename ZST-nthback.rs to zst-nthback.rs

Author: DrAsu33

src/tools/miri/tests/fail/unaligned_pointers/ZST-nthback.rs renamed to src/tools/miri/tests/fail/unaligned_pointers/zst-nthback.rs

@@ -1,7 +1,4 @@
-//@ compile-flags: -Zmiri-symbolic-alignment-check
-//@ run-pass
-
-use std::ptr;
+//@compile-flags: -Zmiri-symbolic-alignment-check
 
 // This is a ZST (Zero-Sized Type) that requires high alignment (alignment 8).
 // The original bug occurred because IntoIter::nth_back tried to calculate an 
@@ -22,6 +19,4 @@ fn main() {
     let v2 = vec![Thing, Thing];
     let mut iter2 = v2.into_iter();
     let _ = iter2.nth_back(0);
-    
-    // The main assertion is that the program runs to completion without Miri reporting UB.
 }