test: disable sigchld test on Windows

Alex Brainman reports that this is the only test
that keeps us from running test/run.

R=alex.brainman, lucio.dere, bradfitz, hectorchu
CC=golang-dev
https://golang.org/cl/4777043

src/run.bash

@@ -93,7 +93,6 @@ done
 ./timing.sh -test
 ) || exit $?
 
-[ "$GOHOSTOS" == windows ] ||
 (xcd ../test
 ./run
 ) || exit $?

test/sigchld.go

+// [ "$GOOS" = windows ] ||
 // $G $D/$F.go && $L $F.$A && ./$A.out
 
 // Copyright 2009 The Go Authors. All rights reserved.