There are others here who are much more knowledgeable than I on this topic, and I'm sure they'll come along shortly, but I'll take a shot:
Unschoolers have gotten into all of the big name Ivy League schools, and hundreds of other colleges/universities as well. There are a variety of ways in which unschoolers choose to ready themselves for college admission including (but not limited to) using textbooks, and finding adults to help them gain the prerequisite knowledge they will need to apply to the colleges of their choice. Many unschoolers are immersed in fields of study in the arts, sciences, agriculture, business, computers, engineering, etc. and often are able to parlay these real life experiences into a unique portfolio which admissions officers are impressed by.
A woman named Cafi Cohen has a website and has published books on the very subject of homeschoolers and unschoolers getting into college. I don't remember the website name, but if you type "Cafi Cohen Homeschool" into google, it will come up.
As far as testing at different levels, that depends upon which state you live in. In California, the state where I live, there are no required tests for homeschoolers ever. In New York, and Pennsylvania, for example, there are required tests at 3 or 4 different grade levels.
What state do you live in?
Laura