Software testing education is becoming increasingly important both in academia and industry. Despite efforts to improve teaching approaches at the university level, many challenges persist for better preparing students for their future careers. In this position paper we present the Testing Robot Challenge tool implementing a serious game designed for motivating the students to practice testing and learn how to write effective unit tests in coverage testing. The game exploits the mechanism of the challenge that students can play against state-of-the-art tools for automated test case generation. It is configurable by teachers, in order to tune the complexity and type of challenges to the specific needs of the students and to the objectives of the course taught. To validate the tool, we performed a preliminary experiment involving 15 students of a Software Engineering course who provided generally positive feedback about it and useful comments for its future improvement.