applets/batterymonitor: thoroughly test the battery list feature

The code mainly contains:

- UPower mock implementation: To support the upower backend and provide
  custom information for testing.
- Private DBus daemon setup: To mock UPower DBus interface, a private
  DBus session has to be set up so the test can register a system bus.
- Test batteries can be listed in the widget, and their names and states
  are correct.