tests: drivers: timer: grtc: Fix GRTC test

The `z_nrf_grtc_timer_get_ticks()` function converts system ticks
to GRTC ticks. It gets the current system tick to calculate an
absolute GRTC value. The same does the test function to provide
an argument to be converted. If the system tick occurs between those
`sys_clock_tick_get()` calls the `z_nrf_grtc_timer_get_ticks()` will
take into account the newer tick while the test estimate bases on
the old tick value. Due to that the maximum result error is 1 system
tick minus 1 GRTC tick which equals (`CYC_PER_TICK` - 1) for GRTC
ticks.

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>