util/deephash: fix unexported time.Time hashing

Updates tailscale/corp#6311

Change-Id: I33cd7e4040966261c2f2eb3d32f29936aeb7f632
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/deephash/testtype/testtype.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testtype contains types for testing deephash.
+package testtype
+
+import "time"
+
+type UnexportedAddressableTime struct {
+	t time.Time
+}
+
+func NewUnexportedAddressableTime(t time.Time) *UnexportedAddressableTime {
+	return &UnexportedAddressableTime{t: t}
+}