There are some great answers in the Quora thread How do you prevent scope creep in agile projects when usability testing? From Todd Zaki Warfel’s answer:

Usability testing doesn’t have to be a formal 3 month engagement. There are plenty of resources out there for doing rapid testing, which can easily be worked into a typical 3-6 week sprint cycle. We do it all the time. Take a week out of the sprint to plan, test, compile recommendations and get to work implementing them. Or as Jeff Gothelf commented, just pick a single day each week to test. You won’t need may participants. If you test weekly, then 3-5 will show a pattern. And anything you don’t catch this week, you’ll catch next week.

From Jeff Gothelf’s answer:

Usability testing needs to be worked in to the sprint timeline. In fact, it should take place during each and every sprint. We test every Thursday. It’s never more than 3 or 4 people. We show whatever we have ready on that day. Sometimes it’s live code and sometimes it’s a paper sketch. Everything in between is fair game. The purpose of this is to get constant feedback, mid-iteration, on the work being designed. In many cases, developers and product owners view portions of these sessions and see the issues that arise. An expectation is built within the team that, based on how things test on Thursday, there may be some tweaks to the scope.