Why do bots only reply in DMs?
You must reply to the channel from which you sent the ‘test’ from the server.
I’d like you to let me know the error.

@client.event
async def on_message(message):
    if message.content == "테스트": # 메세지 감지
        await message.channel.send ("{}, Hello".format(message.author.mention))
        await message.author.send ("{}, User, Hello".format(message.author.mention))