utf16: fix cyclic dependency when testing on Windows

(on Windows, syscall depends on utf16)

R=rsc
CC=golang-dev
https://golang.org/cl/1664053

src/pkg/utf16/utf16_test.go

@@ -2,13 +2,14 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package utf16
+package utf16_test
 
 import (
 	"fmt"
 	"reflect"
 	"testing"
 	"unicode"
+	. "utf16"
 )
 
 type encodeTest struct {