“How did he become a “man” before time began?”
We don’t know; we haven’t been told that. Frankly, such speculation is kinda pointless when there’s plenty I still need to learn about the little stuff, like, oh, faith. For what it’s worth, I think the true nature of the concepts of “time” and “eternity” are a bit beyond mortal man’s grasp.
“Was he a “man” on another planet who “became” God after going through a mortal test on an earth like ours?”
That is not LDS doctrine. It is an opinion held by many Mormons, due to several statements by past leaders. LDS doctrine neither confirms nor denies the idea Again, it’s not necessary for our salvation.
It is not an opinion held by Christians.