Code snippet from https://laracasts.com/discuss/channels/testing/how-to-specify-a-testing-database-in-laravel-5

database.php

<?php
// config/database.php
return [

    // other stuff

    'default' => env('DB_CONNECTION', 'mysql'),

    'connections' => [

        'sqlite_testing' => [
            'driver'   => 'sqlite',
            'database' => ':memory:',
            'prefix'   => '',
        ],

        'mysql' => [
        'driver'    => 'mysql',
        'host'      => env('DB_HOST', 'localhost'),
        'database'  => env('DB_DATABASE', 'forge'),
        'username'  => env('DB_USERNAME', 'forge'),
        'password'  => env('DB_PASSWORD', ''),
        'charset'   => 'utf8',
        'collation' => 'utf8_unicode_ci',
        'prefix'    => '',
            'strict'    => false,
        ],

    ],

    // other stuff

];

TestCase.php

<?php namespace UnitTests;
// tests/TestCase.php

use Illuminate\Support\Facades\Artisan;

class TestCase extends \Illuminate\Foundation\Testing\TestCase
{

    /**
     * Creates the application.
     *
     * @return \Illuminate\Foundation\Application
     */
    public function createApplication()
    {
        putenv('DB_CONNECTION=sqlite_testing');

        $app = require __DIR__ . '/../../bootstrap/app.php';

        $app->make('Illuminate\Contracts\Console\Kernel')->bootstrap();

        return $app;
    }

    public function setUp()
    {
        parent::setUp();
        Artisan::call('migrate');
    }

    public function tearDown()
    {
        Artisan::call('migrate:reset');
        parent::tearDown();
    }

}