There are brothers and sisters, nephews and nieces, neighbor children, who would know the wedding was coming up, and hadn’t yet occurred. You are giving scandal to these little ones regardless if you are chaste or not. That is no small matter, and Christ warns not to scandal little ones!
As for testing the waters, no. When a couple come before God to get married, they are asking God to grant them all the graces they will need to love each other and become holy. If only co-habitating couples could appreciate the grace God wishes to give them, They would see what a deception the enemy tells them, that ‘testing the waters’ will benefit them. It’s a lie and Christians need to be strong in this culture in living in love and the fullness of it. (do you realize that many young girls can’t comprehend having a boyfriend at 16 and being chaste? What a wonderful opportunity for a young couple, especially because they are engaged, to give good example! )
When a couple is married 40 or 50 years, and they can still hold each other and smile, that is not without many efforts on bended knee before God.