add "rackunit" to "mz" distribution
add "rackunit" to "mz" distribution

The `rackunit' library can be used for tests in any code.

The GUI portion of `rackunit' remains in the "plt"
distribution, only.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket