tests: drivers: nrf_rtc_timer: Add a next cycle timeouts test case

Add a test case that schedules a set of consecutive timeouts,
each one on the next cycle of the system timer but each time
after a slightly larger delay.
This test case reveals a problem in the current nrf_rtc_timer
implementation that sometimes a compare value can be missed
what leads to timeouts delayed by 512 seconds.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>