Designing and testing should be done iteratively in a cycle with constant reference and referral to the personas and data. Moreover, user testing should be done at the earliest possible moment the wireframes are finished.
USER GROUP
1 to 2 weekly tests with that group doing various testing exercises to evaluate the UX.
Collate those test results
Re-evaluate the problems and cycle through the process over and over.
You're testing to make sure that the interface reflection of your solution model meets the users expectations - the right stuff in the right place at the right time.