tests: kernel: timer_behavior: Decrease tick rate for nRF

Nordic targets use 24-bit RTC peripheral for system clock. Nordic system
clock timeout implementation relies on RTC CC (capture compare) when
the timeout is in future. Nordic system clock driver allows setting
alarm only to 3 or more counts from current counter value due to silicon
limitation (to ensure that CC event triggers before counter overflow).

RTC CC limitation does not have much impact on normal applications where
there is no need to schedule such short timeouts, but is problematic in
a timer test that expects being able to repeatedly schedule timeouts on
subsequent ticks.

Reduce system tick rate to 8192 on nRF targets to allow setting CC to
the very next tick. With system tick rate being 4 times less than the
hardware tick rate, it is always possible to schedule timeout to happen
in the next tick because ticks are 4 counts apart, i.e. current timer
value + 3 never runs past the next tick.

Fixes: #54211

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>