How can we test functions that aren't exposed when building R packages?

I'm currently developing a graphical analysis package for R. We're trying to use principles from both Clean Code and Test-Driven Development (TDD). But, we've run into a conceptual problem.

How can you test unexposed functions?

Consider the following simplified example. Outer() is a function we're building in our package, and we expose it to users by listing it in the NAMESPACE file. Inner() is a short (~5 line) utility function:

Outer <- function(...) {

  Inner <- function(...) {
    return(x)
  }

  return( Inner() )
}

Most of the user-exposed functions in our package are collections of short, single-behavior Inner() style functions that lie under the umbrella of a single Outer() function the user can call. Granova.ds.ggplot() is one example. Such a modular design is strongly advocated by Clean Code, and we're quite happy with the results. But we don't know how to build unit tests for it, since the functions we want to test aren't accessible outside the scope of the Granova.ds.ggplot() function given how we designed it.

Several ideas/solutions occurred to us:

1. Tests should only have access