CL 5232 added TestServerHijackGetsBackgroundByte, which is failing
on Plan 9, because CloseWrite is not implemented on Plan 9 yet.

Updates #17906.
Updates #18657.

Change-Id: I3c2f73760b0f767f3f9ed2698c855372170e0481
Reviewed-on: https://go-review.googlesource.com/35178Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>