engine: group duplicate test compares site counts

The test compared the copy'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

@@ -6559,6 +6559,7 @@ fn duplicating_a_shape_in_a_group_copies_it_into_the_group() {
         let st = s.doc().unwrap();
         let sites = |id| connect::sites_of(&st.doc, &st.selection, st.shapes(), sid(id)).unwrap();
         let (p, q) = (sites(from), sites(to));
+        assert_eq!(p.len(), q.len(), "{p:?} {q:?}");
         let d = q[0] - p[0];
         assert!(p.iter().zip(&q).all(|(p, q)| (*q - *p - d).hypot() < 1e-6), "{p:?} {q:?}");
         (d.x, d.y)