scripts: Add arg to test_plan.py for alternative test locations

The arg --testsuite-root was copied from twister. When it is used
for test_plan.py it will be propagated to twister calls. This allows
to make alternative test locations (e.g. from another repo) to work
with test_plan.py

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>