Autonomous systems are becoming increasingly popular among industries as well as end-users, and are deployed in numerous tasks. To improve their reliability and to avoid critical failures that impact safety, their testing aims at ensuring that their behavior and decisions are acceptable even in scenarios that have not been foreseen by the developers. As testing of these systems is usually done through field testing, which is costly and is limited in the reproducible scenarios, system-level pre-validation can be done in virtual worlds through simulation, to discover faults and fix them before deploying the system in the real world. However, there is no current standard procedure to conduct simulation-based testing and to ensure satisfying coverage of the most critical scenarios for the system under test (SUT). The aim of this experimental work is to improve and automate the steps of the simulation-based testing related to the generation and selection of test inputs, the exploitation of the results, and the incorporation of dynamic agents in the tests.