You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: modify_column_test.go
    path of file: ./repos/tidb/pkg/ddl
    
    the code of the file until where you have to start completion: // Copyright 2022 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package ddl_test

import (
	"context"
	"fmt"
	"strconv"
	"sync"
	"testing"
	"time"

	"github.co
    
func TestModifyColumnCharset(t *testing.T) {
	store := testkit.CreateMockStore(t)
	tk := testkit.NewTestKit(t, store)
	tk.MustExec("use test")
	tk.MustExec("create table t_mcc(a varchar(8) charset utf8, b varchar(8) charset utf8)")

	result := tk.MustQuery(`show create table t_mcc`)
	result.Check(testkit.Rows(
		"t_mcc CREATE TABLE `t_mcc` (\n" +
			"  `a` varchar(8) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,\n" +
			"  `b` varchar(8) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL\n" +
			") ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin"))

	tk.MustExec("alter table t_mcc modify column a varchar(8);")
	tbl := external.GetTableByName(t, tk, "test", "t_mcc")
	tbl.Meta().Version = model.TableInfoVersion0
	// When the table version is TableInfoVersion0, the following statement don't change "b" charset.
	// So the behavior is not compatible with MySQL.
	tk.MustExec("alter table t_mcc modify column b varchar(8);")
	result = tk.MustQuery(`show create table t_mcc`)
	result.Check(testkit.Rows(
		"t_mcc CREATE TABLE `t_mcc` (\n" +
			"  `a` varchar(8) DEFAULT NULL,\n" +
			"  `b` varchar(8) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL\n" +
			") ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin"))
}