scripts: twister: adaptation for pytest plugin

Making the necessary changes to enable the new pytest plugin.
By default Twister should work without the pytest-twister-harness
plugin installed. To achieve this, each time Twister calls pytest,
the PYTHONPATH environment variable is expanded and the
`-p twister_harness.plugin` option is added to the pytest command.

Co-authored-by: Piotr Golyzniak <piotr.golyzniak@nordicsemi.no>
Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>