Allow unimplemented in other tests

Author: Michael A. Plikk

tests/run-pass/ice_exacte_size.rs

@@ -6,6 +6,7 @@ struct Foo;
 impl Iterator for Foo {
     type Item = ();
 
+    #[allow(unimplemented)]
     fn next(&mut self) -> Option<()> {
         let _ = self.len() == 0;
         unimplemented!()