tests: boards: nrf70: Add ztests for bus library

Add ztests for nrf70 bus lib to help validate functional integrity
of the QPSI/SPI interface to nRF70 device via appropriate reads/writes
from host processor to relevant memory blocks of nrf70 device.
Note that this will NOT serve as a memory test for nrf70 device,
rather just a functional verification of the wiring between the host
processor and the nrf70 device.

Signed-off-by: Bansidhar P.M <bansidhar.mangalwedhekar@nordicsemi.no>