Code review comment for lp:~indicator-applet-developers/unity8/indicator-power-autopilot-test

I've nibbled around the edges of these NFs and have fixed most of the little pieces, but it's past my bedtime so Allan or I will need to finish the revisions tomorrow.

Remaining TODO:

 * device_emulation_scenarios + multiply_scenarios()

 * FakeUPower as a Fixture

 * moving the Indicator helper class to tests/autopilot/unity8/indicators/helpers/indicator_power.py -- not much payoff for this today, but scales if/when we add more indicator helpers

 * GRID_UNIT_PX: is it needed? if so, why?