grilo: Port test-grilo.py to the 0.3 API
Update the test program from the Grilo 0.2 API to the 0.3 one shipped in the Buster-based v2019dev0 release.
- replace Registry.load_plugin_by_id() with Registry.load_all_plugins() and Rregistry.activate_plugin_by_id()
- replace check of MediaBox subclassing with Media.is_container()