laravel phpunit Unknown database ':memory:

This error is bugging me. my laravel version is 5.6 I don't think I have made any changes to my phpunit.xml file except these

<php>
        <env name="APP_ENV" value="testing"/>            
        <env name="CACHE_DRIVER" value="array"/>
        <env name="SESSION_DRIVER" value="array"/>
        <env name="QUEUE_DRIVER" value="sync"/>
        <env name="MAIL_DRIVER" value="array"/>
        <env name="DB_CONNECTION" value="sqlite"/>
        <env name="DB_DATABASE" value=":memory:"/>
        <ini name="display_errors" value="true"/>
    </php>

I'm writing test like this:

<?php

namespace Tests\Feature;

use Tests\TestCase;
use Illuminate\Foundation\Testing\RefreshDatabase;
use Illuminate\Foundation\Testing\DatabaseMigrations;

class DemoTest extends TestCase
{
    use DatabaseMigrations;

    public function test_it_is_nothing()
    {
        $this->get('/volunteers')->assertStatus('200');
    }
}

I have also referenced to this post Laravel multiple databases PHPUnit but not helpful.

via Chebli Mohamed