Testing of autonomous driving systems (ADS) is a crucial yet complex task that requires different approaches to ensure the safety and reliability of the system in all possible scenarios. Currently, there is a lack of understanding of the industry practices for testing such systems, and also the related challenges. To this end, we conducted a secondary analysis of our previous exploratory study, where we interviewed 13 experts from 7 ADS companies in Sweden. We explored testing practices and challenges in industry, with a special focus on scenario-based testing as it is widely used in research for testing ADS. Through a detailed analysis and synthesis of the interviews, we identified key practices and challenges of testing ADS. Our analysis shows that the industry practices are primarily concerned with various types of testing methodologies, testing principles, selection and identification of test scenarios, test analysis, and relevant standards and tools as well as some general initiatives. Challenges mainly include discrepancies in concepts and methodologies used by different companies, together with a lack of comprehensive standards, regulations, and effective tools, approaches, and techniques for optimal testing. To address these issues, we propose a ‘3CO’ strategy (Combine, Collaborate, Continuously learn, and be Open) as a collective path forward for industry and academia to improve the testing frameworks for ADS.