It runs all tests correctly and saves significant time by avoiding the shell script.
However, this is just the code for the command, for review.
A separate CL will move this into the real gotest, which will take some dancing.

R=rsc, peterGo, bsiegert, albert.strasheim, rog, niemeyer, r2
CC=golang-dev
https://golang.org/cl/4281073