tests: timer_behavior: increase stdev tolerance for npcx timer

Change the stdev tolerance stdev from 10 to 33 when npcx timer is used.
This is because the clock source of npcx event timer, which is used to
generate the timeout interrupt, is running at 32.768 KHz.
(i.e. 1 count = ~30.5 us.) The conversion from the absolute system timer
to the event timer count might have -30.5 ~ 30.5 deviation.
The tolerance setting is under the assumption that test sampes are
evenly located at 1030.5 and 969.5 for the worst case.

Fixes #59594

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>