Format the intersect panel test as rustfmt lays it out

The chained lookup in the_intersect_button_joins_a_part_that_intersects was
one line past rustfmt's width, so fmt --check failed on the branch. Split
as rustfmt wants it; no behaviour changes.

ui/dr-ui/src/masks_ui.rs

@@ -1550,7 +1550,12 @@ mod tests {
 
         let index = s.add_mask_part(&id, 2).expect("a part was joined");
         let join = |s: &DevelopSession| {
-            s.masks().get(&id).expect("layer").part(index).expect("part").join
+            s.masks()
+                .get(&id)
+                .expect("layer")
+                .part(index)
+                .expect("part")
+                .join
         };
         assert_eq!(join(&s), Join::Intersect);
         assert_eq!(s.mask_parts(&id)[index].2, 2, "the chip shows ∩");