How do you setup a subdomain?
I did it once in WampServer; i belive its no different to creating a main domain (project) . Never tried this in Laragon but i think the steps would be the same.
You say you've already set up testside.org, so :
Create a new project called, dev.testside.org, use Quick App, this entry goes into your hosts file automatically since you dont have to write to the virtual host file manually. A sub-domain is just an entity of an existing domain name.
As i said i haven't tried this before in Laragon but try it and i think you'll be good to go
unfortunately this does nor work