tests/drivers/nrf_rtc_timer: Fix for nrf52_bsim

Break two busy wait loops with a Z_SPIN_DELAY
(busy_wait() conditionally compiled for the posix arch)
so this test can also be run in the nrf52_bsim.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>