Add regression test for jump in lhs of range

This syntax tree incorrectly gets turned into `return ..=return` when
printed, which is a return whose value is a RangeToInclusive. The
original is a Range whose endpoints are returns.

Author: dtolnay

tests/test_expr.rs

@@ -704,6 +704,8 @@ fn test_fixup() {
         quote! { { (let _ = ()) } },
         quote! { (#[attr] thing).field },
         quote! { (self.f)() },
+        quote! { (return)..=return },
+        quote! { 1 + (return)..=1 + return },
     ] {
         let original: Expr = syn::parse2(tokens).unwrap();