Ensuring the reliability and trustworthiness of connected and automated vehicle (CAV) technologies is crucial before their widespread implementation. Instead of focusing solely on the automation levels of individual vehicles, it is essential to consider the autonomous operations of the entire autonomous transportation system (ATS) to achieve automated traffic. However, designing and generating scenarios that unify the diverse properties of CAV testing and establish mutual trust among stakeholders pose significant challenges. Previous studies have predominantly focused on the automation levels of CAVs when characterizing scenarios, neglecting the autonomous level of the entire scenario from an ATS perspective. Moreover, there remains research potential in evaluating whether the testing scenario libraries can be effectively integrated into the CAV testing process. In this paper, we propose a grading framework for traffic scenarios based on autonomous levels in the ATS. We also classify and summarize the traffic scenarios used in CAV safety testing. Through a comprehensive literature review, we identify prevailing issues and patterns in scenario design and provide insights and directions for future research in this field.