Your test container should use the same Spring configuration as your runtime container. Look at YourProjectTestContainer constructor, it should set the appPropertiesFiles field value to the list containing your app.properties file which in turn defines the cuba.springContextConfig property with the path to the Spring config file.
Is your service accessible in runtime code? In what part of the unit test you invoke AppBeans.get()? Do you define @ClassRule with your container in the test class?