Using ProjectBuilder to test multi-module projects

I am working on a plugin which is intended to be applied to multi-module projects.

But I am unclear how to use ProjectBuilder to assert set-up in these cases. Even basic stuff like having Project#getSubprojects() returning the correct thing (even though I use ProjectBuilder#withParent on the subprojects).

Am I missing something? Or is what I am trying to do just stuck in limbo between ProjectBuilder and GradleRunner?

Or is there a way to access the Project built for GradleRunner? That would work for my purposes.