Adjust tests for newly uplifted cast_ref_to_mut lint

Author: Urgau

tests/fail/modifying_constants.rs

@@ -1,6 +1,8 @@
 // This should fail even without validation/SB
 //@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 
+#![allow(cast_ref_to_mut)]
+
 fn main() {
     let x = &1; // the `&1` is promoted to a constant, but it used to be that only the pointer is marked static, not the pointee
     let y = unsafe { &mut *(x as *const i32 as *mut i32) };

tests/fail/stacked_borrows/shr_frozen_violation1.rs

@@ -1,3 +1,5 @@
+#![allow(cast_ref_to_mut)]
+
 fn foo(x: &mut i32) -> i32 {
     *x = 5;
     unknown_code(&*x);