Testing lens versions is a tricky issue…

I can think of other ways to do this more reliably:

* Using `aug_text_store` to map a snippet to the tree and check how it was mapped. This will require Augeas 1.0.0 and ruby-augeas 0.5.0
* Using a tempfile instead of `aug_text_store`. If we use an instance variable, it will only create the tempfile once per provider, so that might be acceptable.