lib/osutil, lib/scanner: Run symlink test on Windows when possible

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4548

lib/osutil/symlink.go

@@ -0,0 +1,21 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build !windows
+
+package osutil
+
+import (
+	"os"
+)
+
+// DebugSymlinkForTestsOnly is not and should not be used in Syncthing code,
+// hence the cumbersome name to make it obvious if this ever leaks. Its
+// reason for existence is the Windows version, which allows creating
+// symlinks when non-elevated.
+func DebugSymlinkForTestsOnly(oldname, newname string) error {
+	return os.Symlink(oldname, newname)
+}