net/tlsdial: add forgotten test file for go mod tidy

I forgot to include this file in the earlier
7cf8ec8108 commit.

This exists purely to keep "go mod tidy" happy.

Updates #1609

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

net/tlsdial/deps_test.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2021 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.
+
+//go:build for_go_mod_tidy_only
+// +build for_go_mod_tidy_only
+
+package tlsdial
+
+import _ "filippo.io/mkcert"