How to seed database migrations for laravel tests?

All you need to do is make an artisan call db:seed in the setUp function

<?php

use Illuminate\Foundation\Testing\DatabaseMigrations;

class ExampleTest extends TestCase
{
    use DatabaseMigrations;

    public function setUp(): void
    {
        parent::setUp();

        // seed the database
        $this->artisan('db:seed');
        // alternatively you can call
        // $this->seed();
    }

    /**
     * A basic functional test example.
     *
     * @return void
     */
    public function testBasicExample()
    {
        $response = $this->get("https://stackoverflow.com/");

        // ...
    }
}

ref: https://laravel.com/docs/5.6/testing#creating-and-running-tests