tests: tickless_concept: Fix slicing time measurement

The time measurement based on k_uptime_delta() might be not accurate
for some values of CONFIG_SYS_CLOCK_TICKS_PER_SEC. This commit
introduces measurement based on k_cycle_get_32(), which is more
precise.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>