Why do girls test their boyfriends?

My best friend is dating my brother. We went out to a concert last weekend, we had a great time but then she told my brother that she cheated on him at the concert. Which I know is a lie because I was with her the entire time. I confronted her about it and she told me she does this to every guy she dates to "test" them. To see if they'll still stay with her even after she tells them she cheated. Why is she like this? I've also met other people like this to but what's the point?
Why do girls test their boyfriends?
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