suppress dangerous_implicit_autorefs in TB test

Author: RalfJung

tests/pass/tree_borrows/interior_mutability.rs

@@ -1,6 +1,7 @@
 //@revisions: default uniq
 //@compile-flags: -Zmiri-tree-borrows
 //@[uniq]compile-flags: -Zmiri-unique-is-unique
+#![allow(dangerous_implicit_autorefs)]
 use std::cell::{Cell, Ref, RefCell, RefMut, UnsafeCell};
 use std::mem::{self, MaybeUninit};