CL 35234 added TestServerHijackGetsBackgroundByte_big, which is failing
on Plan 9, because CloseWrite is not implemented on Plan 9 yet.

Updates #17906.
Updates #18658.

Change-Id: Icaf3fe3600d586515ecd92aca874104ea81ce6b9
Reviewed-on: https://go-review.googlesource.com/35179
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>