Add test of attr precedence inside subexpression

    ---- test_fixup stdout ----
    thread 'test_fixup' panicked at tests/test_expr.rs:716:9:
    original: (# [attr] thing) . field
    reconstructed: # [attr] thing . field

tests/test_expr.rs

@@ -702,6 +702,7 @@ fn test_fixup() {
         quote! { (..) += () },
         quote! { (1 < 2) == (3 < 4) },
         quote! { { (let _ = ()) } },
+        quote! { (#[attr] thing).field },
     ] {
         let original: Expr = syn::parse2(tokens).unwrap();