I'd like to know the testing method used to determine that ;)

It's not science, it's marketing. :)

Wikipedia says, "The quokka has no fear of humans and it is common for it to approach them closely."

Some marketer saw that lack of fear and their "smile" and called it happiness:

Definitely the smile ;)

Funny how anthropomorphizing an animal makes it seem closer to us. 

They will tolerate petting if you give them a banana: