add test with an Rc cycle to create a memleak

tests/compile-fail/memleak_rc.rs

@@ -0,0 +1,12 @@
+//error-pattern: the evaluated program leaked memory
+
+use std::rc::Rc;
+use std::cell::RefCell;
+
+struct Dummy(Rc<RefCell<Option<Dummy>>>);
+
+fn main() {
+    let x = Dummy(Rc::new(RefCell::new(None)));
+    let y = Dummy(x.0.clone());
+    *x.0.borrow_mut() = Some(y);
+}