The role of the tester usually goes along the lines of:
You write the code, and then I test it.
What’s wrong with that statement? Essentially you’re missing a great deal of the skillset that a tester has. A tester will often look to break a system when doing exploratory testing, searching out scenarios and using their experience to point out where functionality, security or performance is poor. There’s great value in this up front! The acceptance criteria, or amending the definition of done, if it applies to all stories, is an area where the tester can add a lot of value. The product owner may know their product really well but the tester often knows the system in greater detail. So make sure your testers are involved in writing the acceptance criteria and actively helping the product owner out. Don’t forget your NFR’s too.