ngotest: a new gotest command, written in Go.

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