Do cabin evacuation tests include passengers with limited mobility?
As @mins said in the comments, the 90 seconds is a limit for a evacuation test. Having equal test requirements means that all planes have a minimum, similar evacuation capability.

That said, the conditions of an evacuation test are rather benign: people participating in them know nothing is really wrong, there are no hazards like smoke or fire, and the plane is in a normal attitude (no broken gear or hull, engines stopped). See 
                
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In real emergencies, the time to evacuate may be greater than 90 seconds, and the crew and ultimately the captain has the responsibility to ensure that everyone has been evacuated safely and leave the ship the last. So, crew incapacitation left apart, you wouldn't be left behind.