[parser] RestoreStringWithoutCharset and RestoreStringWithoutDefaultCharset flags not work

[chenlujjj]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

import (
	"fmt"
	"os"
	"strings"

	"github.com/pingcap/tidb/parser"
	"github.com/pingcap/tidb/parser/ast"
	"github.com/pingcap/tidb/parser/format"
	_ "github.com/pingcap/tidb/parser/test_driver"
)

func main() {
	rawSql := "SELECT 'a'"
	restoreCtx := format.NewRestoreCtx(format.RestoreStringSingleQuotes|format.RestoreStringWithoutCharset|format.RestoreStringWithoutDefaultCharset, os.Stdout)

	astNode, err := parse(rawSql)
	if err != nil {
		panic(err)
	}

	err = (*astNode).Restore(restoreCtx)
	if err != nil {
		panic(err)
	}
}


func parse(sql string) (*ast.StmtNode, error) {
	p := parser.New()

	stmtNodes, _, err := p.Parse(sql, "", "")
	if err != nil {
		return nil, err
	}
	return &stmtNodes[0], nil
}

2. What did you expect to see? (Required)

Output: SELECT 'a'

3. What did you see instead (Required)

SELECT _utf8mb4'a'

4. What is your TiDB version? (Required)

[Defined2014]

The test_driver file only be used in UT, so change it to severity/minor.