Come dressed in a mature way.
Hopefully frum boys do not dress this way anyway, but teenage boys who come to the test in washed out ragged jeans and a t-shirt and look like a “dumb teenager” are likely to be watched more closely and “graded” more harshly than if they were wearing a nice pair of slacks and a button down shirt.
The maturity with which you dress may be taken as a reflection of the maturity with which you will drive.