You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: handler_test.go
    path of file: ./repos/iris/hero
    
    the code of the file until where you have to start completion: package hero_test

import (
	"fmt"
	"testing"

	"github.com/kataras/iris/v12"
	. "github.com/kataras/iris/v12/hero"
	"github.com/kataras/iris/v12/httptest"
)

// dynamic func
type testUserStruct struct {
	ID       int64  `json:"id" form:"id" url:"id"`
	Username string `json:"username" form:"username" url:"username"`
}

func testBinderFunc(ctx iris.Context) testUserStruct {
	i
    
func TestBindFunctionAsFunctionInputArgument(t *testing.T) {
	app := iris.New()
	postsBinder := func(ctx iris.Context) func(string) string {
		return ctx.PostValue // or FormValue, the same here.
	}

	h := New(postsBinder).Handler(func(get func(string) string) string {
		// send the `ctx.PostValue/FormValue("username")` value
		// to the client.
		return get("username")
	})

	app.Post("/", h)

	e := httptest.New(t, app)

	expectedUsername := "kataras"
	e.POST("/").WithFormField("username", expectedUsername).
		Expect().Status(iris.StatusOK).Body().IsEqual(expectedUsername)
}