Document how to use pytest_root keyword. Update docs
with changes in pytest-twister-harness plugin.
Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>
Add DeviceAbstract class to default imports from pytest-twister-harness
package to simplify importing DUT package, when creating tests.
Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>