The default policy currently directly references the private
variable next_event from policy_events.c to then convert the cycle
of said event (if exists) to a kernel tick in the future, something
policy_events.c already implements and exposes through
pm_policy_next_event_ticks().
Additionally, the implementation of pm_policy_next_state() in
policy_default.c already gets the nearest kernel tick, wherein
the next event has already been accounted for in, see
implementation of pm_system_suspend().
This commit removes the redundant and layer violating computation
if the tick of the next event from policy_default.c and updates
the test test_pm_policy_events to not use default policy to
determine if pm_policy_next_event_ticks() is correct.
Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>