I just wrote how you can use inheritance with MiniTest: Liskov Principle & MiniTest.

It's pretty easy to extend MiniTest. Below I show you how to add very simple implementation of expect {}.to change {}.by(x) from RSpec, in 13LOC.
It should work out of the box on Ruby 1.9.
https://gist.github.com/2584768

I just launched EmbeddedRecord, a gem similar to what was done in RailsCasts #189 Embedded Association.