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 <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: David du Colombier <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/net/http/httputil/reverseproxy_test.go

@@ -23,6 +23,7 @@ import (
 	"net/url"
 	"os"
 	"reflect"
+	"runtime"
 	"slices"
 	"strconv"
 	"strings"
@@ -1560,6 +1561,11 @@ func TestReverseProxyWebSocketHalfTCP(t *testing.T) {
 	// - that closing the write stream is propagated through the proxy and results in reading
 	//   EOF at the other end of the connection
 
+	switch runtime.GOOS {
+	case "plan9":
+		t.Skipf("not supported on %s", runtime.GOOS)
+	}
+
 	mustRead := func(t *testing.T, conn *net.TCPConn, msg string) {
 		b := make([]byte, len(msg))
 		if _, err := conn.Read(b); err != nil {