You really, really shouldn't arrive late. If you don't show up and miss check in, your test center could state you're a no show. Should that happen, your test is automatically voided and you have to reschedule.
Here's the section from MCAT Essentials:
Make sure you get to the testing center 30 minutes before your scheduled start. The Test Center Administrator will begin checking in examinees a half hour before the appointment time on your confirmation. They will give you a locker and key for your personal items, verify your identification, and help you get settled into your workstation. For many reasons, some examinees miss their start times. If this happens to you it happens to the best of us go into the registration system 48 hours after the test date and time, to register and pay for another test. There are no refunds for missed exams (no shows).