Switch a test case to the "utf-8" spelling.

The spelling "utf8" is not recognized by CCL.
Another encoding and external formats fix for portability.

Some of our internal values now depend on the implementation, and could
either be a symbol on SBCL or an external-format structure on CCL. We
could typecase our way out I suppose, but it might be that SBCL has a
different version of the external-format type, so we'd rather use #+.