tests: kernel: timer: timer_api: Tweak test_timer_remaining

Test test_timer_remaining is comparing time captured by k_busy_wait and
k_timer_remaining_ticks. Test was accepting 1 tick difference between
those two. If system clock frequency is low (e.g. 100 Hz) 1 tick is a
long time but if system clock is high then 1 tick may not cover for
processing latency. Instead of using fixed 1 tick test is now converting
100 us to ticks (ceiling) to cover for cases where system clock is high.
100 us processing latency time is an arbitrary value.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>