test/playlist: unit tests for the playlist plugins

Only "pls" for now.

src/io/StringOutputStream.hxx

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// author: Max Kellermann <max.kellermann@gmail.com>
+
+#pragma once
+
+#include "OutputStream.hxx"
+#include "util/SpanCast.hxx"
+
+#include <string>
+
+class StringOutputStream final : public OutputStream {
+	std::string value;
+
+public:
+	const std::string &GetValue() const & noexcept {
+		return value;
+	}
+
+	std::string &&GetValue() && noexcept {
+		return std::move(value);
+	}
+
+	/* virtual methods from class OutputStream */
+	void Write(std::span<const std::byte> src) override {
+		value.append(ToStringView(src));
+	}
+};