Many driving tests are failed on one-way systems due to learner drivers using the incorrect lane, realising they are in the incorrect lane and trying to swerve across without making sure that its actually safe to.
Driving tests are not failed due to using the incorrect lane providing it is legal and safe.
If you find yourself in the wrong lane, try and access the correct lane only if it is safe and legal to do so.
You will need to use the Mirror Signal Position routine and check over your right shoulder before changing lanes.
Check road markings also as solid continuous white lines do not permit crossing.
If you cannot access the correct lane, remain in your current lane and simply go in that direction
the examiner will take you on an alternative route and you won't fail your driving test for going the wrong way safely.