Adjust some tests for invalid_reference_casting improvements

Author: Urgau

tests/fail/both_borrows/illegal_write1.rs

@@ -1,6 +1,8 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 
+#![allow(invalid_reference_casting)]
+
 fn main() {
     let target = Box::new(42); // has an implicit raw
     let xref = &*target;

tests/fail/stacked_borrows/illegal_write3.rs

@@ -1,3 +1,5 @@
+#![allow(invalid_reference_casting)]
+
 fn main() {
     let target = 42;
     // Make sure raw ptr with raw tag cannot mutate frozen location without breaking the shared ref.