net/http/httputil: skip new test on Plan 9

net/http/httputil: skip new test on Plan 9

TestReverseProxyWebSocketHalfTCP requires half closed connections,
which aren't supported on Plan 9.

For #35892
Fixes #72095

Change-Id: I64b458bc15ac3b8eda43dc871bf67ada32a59708
Reviewed-on: https://go-review.googlesource.com/c/go/+/654636
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>