raco test: add history note on `--heartbeat`
raco test: add history note on `--heartbeat`

Should have been included with f3c8638366.

original commit: 2d3da47447bd91849c8e8e1981cb8f47278dade7

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