net/http/cgi: fix plan9 build

Cleanup CL https://golang.org/cl/41691 broke the plan9 build by removing
a use of a package but not removing the package import.

Trybots don't check that. I filed #20119 for that.

Change-Id: Ia030e6924665dfb871ca964455b899d51b0200c2
Reviewed-on: https://go-review.googlesource.com/41752Reviewed-by: David du Colombier <0intro@gmail.com>

src/net/http/cgi/plan9_test.go

@@ -9,7 +9,6 @@ package cgi
 import (
 	"os"
 	"strconv"
-	"testing"
 )
 
 func isProcessRunning(pid int) bool {