fix: polish the tests

Author: zhjwpku

src/iceberg/test/json_internal_test.cc

@@ -110,6 +110,7 @@ TEST(JsonInternalTest, SortOrder) {
   SortField st_ts(5, identity_transform, SortDirection::kAscending, NullOrder::kFirst);
   SortField st_bar(7, identity_transform, SortDirection::kDescending, NullOrder::kLast);
   ICEBERG_UNWRAP_OR_FAIL(auto sort_order, SortOrder::Make(100, {st_ts, st_bar}));
+  EXPECT_TRUE(sort_order->Validate(*schema));
   nlohmann::json expected_sort_order =
       R"({"order-id":100,"fields":[
           {"transform":"identity","source-id":5,"direction":"asc","null-order":"nulls-first"},

src/iceberg/test/sort_order_test.cc

@@ -259,10 +259,10 @@ TEST_F(SortOrderTest, MakeUnboundSortOrder) {
   SortField sort_field_invalid(999, Transform::Identity(), SortDirection::kAscending,
                                NullOrder::kFirst);
 
-  auto sort_order =
-      SortOrder::Make(1, std::vector<SortField>{*sort_field1_, sort_field_invalid});
-  ASSERT_THAT(sort_order, IsOk());
-  auto validate_status = sort_order.value()->Validate(*schema_);
+  ICEBERG_UNWRAP_OR_FAIL(
+      auto sort_order,
+      SortOrder::Make(1, std::vector<SortField>{*sort_field1_, sort_field_invalid}));
+  auto validate_status = sort_order->Validate(*schema_);
   EXPECT_THAT(validate_status, IsError(ErrorKind::kInvalidArgument));
   EXPECT_THAT(validate_status,
               HasErrorMessage("Cannot find source column for sort field"));