Add test of parentheses needed in call of field

    ---- test_fixup stdout ----
    thread 'test_fixup' panicked at tests/test_expr.rs:717:9:
    original: (self . f) ()
    reconstructed: self . f ()

tests/test_expr.rs

@@ -703,6 +703,7 @@ fn test_fixup() {
         quote! { (1 < 2) == (3 < 4) },
         quote! { { (let _ = ()) } },
         quote! { (#[attr] thing).field },
+        quote! { (self.f)() },
     ] {
         let original: Expr = syn::parse2(tokens).unwrap();