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Well, Doug Rattman made a good choice by making Chell the test subject. (On her file, he saw that Chell was really stuborn. she didn;t even tell the scientists what her last name was).
It would also make for a really crappy game if Chell died in the first couple of Chambers. It's just a game. If Gordon Freeman was like the other scientists, Half-Life would have been a really short game as well.
They're rare humans with the ability to rewind time if they die, to try again with the memories of their previous selves. This is the ability that GLaDOS says makes a good test subject, and one in a million people has it. 
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Hey, you're the one who asked if they had superhuman abilities. And there are some points where the best way to solve the test involves a bit of trial and error, how would they get that right the first time? 
That's a whole other discussion about the lack of physics and superhuman speed.
But she may have some flaws.
She can survive one or two hits by lasers or turrets, but she is instantly taken out by deadly goop.
And how does she get hurt by seemingly infinite falls? She has the boots.
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