It was never tested and also breaks Windows.

run.go doesn't yet support the proper !windows,!plan9 syntax.

««« original CL description
test: do not run SIGCHLD test on Plan 9

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10017045
»»»

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/10024044