disable Rc test for now, it ain't working

Author: RalfJung

tests/run-pass/rc.rs

@@ -1,5 +1,5 @@
-// FIXME: Validation disabled due to https://github.com/rust-lang/rust/issues/54908
-// compile-flags: -Zmiri-disable-validation
+// FIXME: Disabled due to https://github.com/rust-lang/rust/issues/55747
+// ignore-test
 
 use std::cell::RefCell;
 use std::rc::Rc;