test/playlist: unit tests for the playlist plugins

Only "pls" for now.

test/playlist/PlaylistUtil.hxx

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright The Music Player Daemon Project
+
+#pragma once
+
+#include <memory>
+#include <string>
+
+class SongEnumerator;
+
+std::unique_ptr<SongEnumerator>
+ParsePlaylist(const char *uri, std::string_view contents);
+
+std::string
+ToString(SongEnumerator &e);