power_domain: additional UTs to the PD

This patch includes further unit tests for the primary
power domain module file.

These will target initiating and processing a power state
transition, completing system suspend, get state requests
and get power domain information.

Signed-off-by: Katherine Vincent <katherine.vincent@arm.com>
Change-Id: I703375b66b5e5695b7afc6671527b99720749c5e