Improve the coverage of key path in pkg/restore

## Development Task

- [ ] [db.go#L107](https://codecov.io/gh/pingcap/br/src/master/pkg/restore/db.go#L107) what if `create table statement` contains comment like  `/*xxx*/ CREATE /*xxx*/ TABLE`

- [ ] [import.go#L217](https://codecov.io/gh/pingcap/br/src/master/pkg/restore/import.go#L217)

