I am build a Telegram Bot now and are testing it manually with a Telegram Client.
Is there a way I can send client messages in the same way I can build bots?
Alternatively you can use an MTProto Telegram user account library like MadelineProto in PHP, Telethon and Pyrogram in Python 3. You can use these library to automate an user account to test your bot.
Just to note that you might hit the flood ban if you send message too frequently.