Think there's an error in thinking that the aerodynamic test aircraft has an interior, and thus exits, like a normal airliner. Our RJs have a modifed lower access hatch forward that is hydraulically powered to open against the airflow & act as a blast diverter. The cabin floor opens to provide a smooth slide out. The aft baggage door is also powered such that it can track inboard to provide an aft means of escape. Some smaller biz jets have large sections of the fuselage able to be blown out explosively with, again, a blast door arrangement to get you safely through the boundary layer.
The pressurisation system is also modified to allow full emergency drepress to ambient not to an intermediate cabin altitude plus, of course, gaseous oxygen for all. Any hazardous testing limits the crew (normally to three, two pilots & an FTE).
FYI it's not just test aircraft that have bail out doors - the KC-135 has a similar facility just at the rear of the cockpit.