net/http: skip TestServerHijackGetsBackgroundByte on Plan 9

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>

src/net/http/serve_test.go

@@ -5180,6 +5180,9 @@ func TestServerDuplicateBackgroundRead(t *testing.T) {
 // bufio.Reader.Buffered(), without resorting to Reading it
 // (potentially blocking) to get at it.
 func TestServerHijackGetsBackgroundByte(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{})