tsunamis, or cholera outbreaks, or pediatric cancer. An all-powerful being could design a test of our moral fiber/character that doesn't have those things as a prerequisite.
I'm not sure why we're hung up on this though, because again, that's a more traditionally Christian view of God that conflicts explicitly with lds teaching from the Book of Mormon. Alma 42 is clear on things god cannot do or else he would cease to be god. That makes him definitionally not an all-powerful being.