websocket: skip TestClose on Plan 9

Updates golang/go#11454.

Change-Id: Ib3762897b15d47d42002aafcdb1f634504c5d82c
Reviewed-on: https://go-review.googlesource.com/12889
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

websocket/websocket_test.go

@@ -426,6 +426,10 @@ func (c *closerConn) Close() error {
 }
 
 func TestClose(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/11454")
+	}
+
 	once.Do(startServer)
 
 	conn, err := net.Dial("tcp", serverAddr)