The namespace shall also be used for testing the rendering of
`Container`s and `Element`s (including the `Scrollable` element).
The testing renderer currently is a striped down version of the double
buffered render without the secondary buffer and the flushing to stdout.
The internal `Cell` slice (the *screen*) is used for equality checks.
The testing namespace shall provide a way to describe the expected
`Cell` slices that should be validated against.
