Improving unit tests for _do_sync_power_state

Improve the unit tets coverage, and tightening up existing
tests surrounding the _do_sync_power_state method.

Change-Id: I9fd4c4c42936fcf8fc4f36bf35b6e691c495b8b0
Closes-bug: #1297404
Merge "Improving unit tests for _do_sync_power_state"