net/http: skip TestServerHijackGetsBackgroundByte_big on Plan 9

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>

src/net/http/serve_test.go

@@ -5245,6 +5245,9 @@ func TestServerHijackGetsBackgroundByte(t *testing.T) {
 // immediate 1MB of data to the server to fill up the server's 4KB
 // buffer.
 func TestServerHijackGetsBackgroundByte_big(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping test; see https://golang.org/issue/18657")
+	}
 	setParallel(t)
 	defer afterTest(t)
 	done := make(chan struct{})