tests: kernel: tickless: tickless_concept: Support more frequencies

Test was assuming that CONFIG_SYS_CLOCK_TICKS_PER_SEC=100 but such
frequency is not well supported on SoC which are based on 32768 Hz RTC
clock. Align test to be able to work with different sys_clock
frequency by converting previously 20 ticks to milliseconds.

Set CONFIG_SYS_CLOCK_TICKS_PER_SEC=32768 when system clock is using
Nordic RTC timer driver.

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