Splitting the file into a main/test would have been a bad idea due to the smart
inlining with the standard's text.

The previous solution to always load the file is also bad for performance.

The hopefully final solution is to create an rfc2109.test system and before loading
rfc2109.lisp in it rebind *features* and push a symbol (:test) into it. The main
lisp file #+test for this symbol and defines the tests when it's loaded through
rfc2109.test.

darcs-hash:b912ce8b527d112d4aa0f74770b5eec3d8067a78