Why did Joseph never tell anyone that his brothers had sold him?

In the Testament of Joseph, Joseph says he kept silent so that he would not put his brothers to shame. When the Ishmaelites questioned him, he told them he was a slave from their own house, even when their leader said his looks proved otherwise and threatened him with death. In Egypt, Potiphar had him stripped and beaten for giving the same answer. Later the Ishmaelites returned, having learned that he was the son of a powerful man in Canaan whose father was mourning him in sackcloth. Joseph says he wanted to weep but held back and again said he was a slave. He even stayed quiet when a eunuch paid eighty pieces of gold for him and reported a hundred, so that the man would not be punished. Joseph draws a lesson from this for his children: love one another and patiently hide each other's faults, because God delights in the unity of brothers. He adds that when his brothers came to Egypt he did not reproach them, and that after Jacob's death he loved them even more.