Add regression test for macro expansion

Author: tesuji

tests/ui/toplevel_ref_arg.rs

@@ -19,6 +19,8 @@ fn main() {
     let (ref x, _) = (1, 2); // ok, not top level
     println!("The answer is {}.", x);
 
+    let ref x = vec![1, 2, 3];
+
     // Make sure that allowing the lint works
     #[allow(clippy::toplevel_ref_arg)]
     let ref mut x = 1_234_543;