Hardly you will find such documentation which guides you to test the odoo functionally. It's mainly done based on our experience and the wisdom. 

still, I would like to share my experience on how I am learning and testing the odoo functionally. 

I use to stay updated with the question asked in the odoo forum and stack overflow. Try my best to get there answer which help me to elaborate my knowledge in practical cases. I found the cases to learn the odoo functionality. 

Secondly, keep checking this URL here odoo gives the best sources for functional knowledge   https://www.odoo.com/event