Developers shouldn't test, they should throw it over to QA who will test it precisely to meet the defined requirements.

The Product Manager's job is to communicate the customers needs to the developers/designers and the developers/designers constraints back to the customers.

It's up to the developers and designers to understand those constraints and make sure they are communicated back.

Ive observed a modern trend of little to no QA. Managers and CTOs insist developers can test their own systems. Maybe this makes more sense in the early phases of product development where I find myself lately? Seems to capture a lot of dev's time.