All i do to help me is just before i start to reverse, i set my right mirror downwards so i can how close i am to the kerb. This helps me to get as close as possible without hitting it, and before i drive off i just set it back to normal.
would this be okay to do in the test, adjusting your mirror for a turn in the road?
Would you do that in real life where traffic is waiting for you on both sides?
It takes too long to be faffing and fannying about with mirrors in the middle of a road mid-manoeuvre while traffic is waiting. Even worse if you have to do it twice: once before the 2nd shunt and again before the 3rd. If you don't clear the kerb on the 3rd shunt, are you going to spend another minute blocking traffic while you lower it again for the 4th and raise it again for the 5th?
To answer your question about "test", you will be faulted if you take an unreasonably long time to complete the manoeuvre.
You should have sufficient judgement skills to judge the size of your car and proximity to the kerb without turning your mirrors down each time. Ask your instructor to train your judgement.