Add test for Apertis update manager error conditions

Add test to check proper behavior in the following circumstances:
- Power cut during an update
- USB mass storage unplug during an update
- The disk is full during an update

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>