organize compile-fail tests in folders

tests/compile-fail/dangling_pointers/deref-invalid-ptr.rs

@@ -2,6 +2,6 @@
 // compile-flags: -Zmiri-disable-validation
 
 fn main() {
-    let x = 2usize as *const u32;
-    let _y = unsafe { &*x as *const u32 }; //~ ERROR invalid use of 2 as a pointer
+    let x = 16usize as *const u32;
+    let _y = unsafe { &*x as *const u32 }; //~ ERROR invalid use of 16 as a pointer
 }