Autonomous ships represent a transformative advancement in maritime transportation, promising significant shifts in cargo and passenger transport from road to sea, reduction in operational costs, minimized risks for crew, and lower environmental impacts. As a critical emerging infrastructure, their safety is a subject of ongoing debate. This paper explores the safety assurance of autonomous ships with a particular focus on compliance with the International Regulations for Preventing Collisions at Sea (COLREG). Central to this study is the development of a sophisticated virtual testing environment, supported by a digital twins that replicates key aspects of environment and autonomous vessels, including sensors and actuators. This virtual environment simulates complex navigational scenarios to rigorously test and validate the autonomous navigation systems against COLREG standards under varied maritime conditions. The solution integrates a dynamic test management system that not only orchestrates simulations but also automatically generates and assesses test scenarios to identify potential deficiencies in vessel performance.