It's not explicitly said, but maybe you remember that Ciri didn't go through the mutations or the Grass Test because none of the witchers, not even Vesemir, knew what could happen in the organism of a girl with the mutations. You can say that maybe (MAYBE) in the past could be female witchers though it's never mentioned, before the attack to Kaer Morhen, when witchers still have the total knowledge of the mutations thing, but in Geralt's time that would be nearly impossible. I don't think the witchers would risk innocent human lives making experiments just to allow girls to go through the Grass Test.Plus, I remember that in one of the short stories, an old lady describes the witchers as monsters, as they were included in her (really old) book of monsters, and in the description read on the book it was said, aprox, that witchers were lustful men, or something like that.