Hi @JuLog,
if you use Spring Boot, I recommend to use the engine as a Spring bean instead of using the ProcessEngineRule. The rule actually tries to spawn a new in memory engine based on the configuration xml file. It also seems that you are using some old version, since you are using an activiti.cfg.xml instead of a camunda one.
You can have a look at this sample project to get started with a working test:
Best,
Ben