UPDATE IN PROGRESS: Placeholder - revised assignment still in progress.
Throughout the development of your team's game code and assets, your team should periodically playtest your game in progress to ensure newly-added features are functioning as intended — and to identify any issues that should be fixed.
Your team's Programming Lead should be testing the game as new code and assets are added. However, it will be helpful to have your team's Art Lead or Research Lead act as another playtester:
Your team's Art Lead could playtest the game to verify that newly-added assets (animated sprites, sound effects, etc.) are functioning as intended. For example, are the sprites the correct size, are the sprite's animations playing properly, etc.
Your team's Research Lead could playtest the game more thoroughly — by not only testing new features but also ensuring existing features still work.
Communication between your team members will be important, so the internal playtesting can focus on specific game features that need to be tested, while also providing specific feedback on any issues that are detected.