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/18578Reviewed-by: Chris Broadfoot <cbro@golang.org>