ITE tests/kernel/sleep: tune CONFIG_SYS_CLOCK_TICKS_PER_SEC down

We need more time to run codes because of the performance,
so I tune CONFIG_SYS_CLOCK_TICKS_PER_SEC down to reduce
the times of running k_usleep(1), then it can pass test_usleep().

Verified by follow test pattern:
west build -p always -b it8xxx2_evb tests/kernel/sleep

fixes #46208

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>