scripts: add pytest plugin

Adding pytest plugin dedicated to running pytest tests in Zephyr
project. This plugin provides a dut fixture which allows to handle
bidirectional communication with the device under test. This version
of plugin can be used for tests dedicated to real hardware, QEMU and
native_posix simulator.

Co-authored-by: Lukasz Fundakowski <lukasz.fundakowski@nordicsemi.no>
Co-authored-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>
Co-authored-by: Katarzyna Giadla <katarzyna.giadla@nordicsemi.no>
Signed-off-by: Piotr Golyzniak <piotr.golyzniak@nordicsemi.no>