Lucas says he always meant for it to be hereditary to some extent, otherwise it wouldn't have mattered who Luke's father was from the get go. The dynamic that Luke was special because of who his father was is evident even in ANH. Otherwise how could the Jedi be made all but extinct?

The Sith had been in super secret hiding for generations. Of course they would be able to get around having a sith candidate tested. Even if they didn't create a baby of their own, they would have no problem stealing a baby before he was tested or even after he had been tested.

From what I understand, Maul was raised from near birth as a Sith.