Right now referencing the constant `AbstractController::Rendering`
causes `ActionView::Base` to be loaded, and thus the load hooks for
action_view are run. If that load hook references any part of action
view that then references action controller (such as
`ActionView::TestCase`), the constant `AbstractController::Rendering`
will attempt to be autoloaded and blow up.

With this change, `ActionView::LoadPaths` no longer requires
`ActionView::Base` (which it had no reason to require). There was a
needed class from `AbstractController::Base` in the Rendering module,
which I've moved into its own file so we don't need to load
all of `AbstractController::Base` there.

This commit fixes
https://github.com/rails/rails-controller-testing/issues/21