context/ctxhttp: don't test on plan9

plan9's network deadline & cancelation code is so brittle that it's
not even worth testing this package there. It only results in 10
minute deadlocks and slows down the trybots which should be ~30
seconds, not 10 minutes.

Change-Id: I097a885ae5887f0ecf357611aa7fae8885e18e05
Reviewed-on: https://go-review.googlesource.com/18578
Reviewed-by: Chris Broadfoot <cbro@golang.org>

context/ctxhttp/ctxhttp_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !plan9
+
 package ctxhttp
 
 import (