What would you test for?

If a home is already there, and the foundation is a standard foundation and holding up, I'd say you are just fine if talking about soil load bearing features.

Are there problems with foundations and building movement in the neighborhood? With adjacent homes? With the home you are considering purchasing? Why are you asking about soils tests?

We purchased a tear down house and did a soil test. We are in N. Texas and have clay soil that shifts a lot depending on moisture content. It was highly recommended for us because we knew we were going to be building a pier and beam home and wanted to know how far down we'd have to drill before hitting rock. Our structural engineer who designed our foundation needed that info. And subsequently the foundation contractors needed that design to give us an accurate bid. Apparently 18 foot deep piers cost more than 12 foot deep piers. :)

If we were going to do a more common post tension slab foundation, I don't believe we would have needed to a soil test especially since there was a house there before and there is nothing else exceptional about the lot (flat).