International maritime traffic is controlled by collision-avoidance regulations (COLREGs) with 41 standardized rules describing how a vessel should navigate in the proximity of other vessels. Since some rules can be overridden by human judgement when resolving critical encounters of vessels, justifying COLREGs compliance has become a significant challenge in the increasing presence of autonomous surface vehicles (ASVs) operated without (or with only remote) human control. This paper provides a high-level framework and long-term research agenda towards the automated synthesis of test scenarios to assure COLREGs compliance for ASVs by exploiting various model-driven engineering techniques. By adapting ideas from testing self-driving cars, we envisage a multi-layered test scenario generation approach involving functional, logical and concrete scenarios. In the current paper, we demonstrate how functional scenarios of COLREGs situations between given vessels can be precisely formalized by using metamodels, domain-specific graph models and first-order logic graph constraints. By using automated model generation techniques, we derive a complete set of functional-level test scenarios, which includes all possible COLREGs situations that may arise between given vessels. As initial result, we provide several dangerous situations involving only three vessels where a potential collision may occur even when all vessels follow the COLREGs, which showcases that some COLREGs rules need further clarification for the safe regulation of ASVs.