make `--run-if-absent' the default mode for `raco test'
make `--run-if-absent' the default mode for `raco test'

This convention makes it easier to deal with a set
of ".rkt" files that implement tests, while a `test'
module implements a `main'-like split for some of the
files.

original commit: 63a4414863eb69da7597a539102e020a2df8f590

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