Add a simple test for external renderer (#20033)

Fix #16402

tools/external_renderer.go

@@ -0,0 +1,23 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// gobuild: external_renderer
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	if len(os.Args) > 1 {
+		fmt.Print(os.Args[1])
+	} else {
+		_, err := io.Copy(os.Stdout, os.Stdin)
+		if err != nil {
+			fmt.Println(err)
+		}
+	}
+}