engine: group copy test compares site counts

The test compared the pasted shape's connection sites with the original's
through zip, which stops at the shorter list. It now checks that both have
the same number of sites first.

crates/engine/src/tests.rs

@@ -269,6 +269,7 @@ fn a_shape_copied_from_a_group_pastes_where_it_was_drawn() {
             assert!(s.doc().unwrap().shapes().iter().any(|x| x.id == pasted), "case {n}: {cmd} pastes on the top level");
             let d = if same_slide { off } else { 0.0 };
             let now = sites(&s, pasted);
+            assert_eq!(was.len(), now.len(), "case {n}: {cmd}");
             assert!(
                 was.iter().zip(&now).all(|(p, q)| (q.x - p.x - d).hypot(q.y - p.y - d) < 1e-6),
                 "case {n}: {cmd} on the same slide {same_slide}: drawn at {was:?}, pasted at {now:?}"