Why PHP unit test is unable to locate factory for [App\Reply]?

When I run the same test of Feature test,it passes the test. But when I run the same test on Unit test, It gives me {InvalidArgumentException: Unable to locate factory for [App\Reply]}. Here is the My test

  <?php

namespace Tests\Unit;

use Illuminate\Foundation\Testing\RefreshDatabase;
use PHPUnit\Framework\TestCase;

class ReplyTest extends TestCase
{
use RefreshDatabase;

/** @test */
public function a_reply_has_user()
{
    $reply = factory("App\Reply")->create();
    $this->assertInstanceOf("App\User",$reply->user);
}
}

Here is the Error

1) Tests\Unit\ReplyTest::a_reply_has_user
InvalidArgumentException: Unable to locate factory for [App\Reply].

Here is the factory for the Reply:

<?php

/** @var \Illuminate\Database\Eloquent\Factory $factory */

use App\Reply;
use Faker\Generator as Faker;

$factory->define(Reply::class, function (Faker $faker) {
    return [
        "user_id"=>factory("App\User")->create(),
        "thread_id"=>factory("App\Thread")->create(),
        "body"=>$faker->sentence
    ];
});