Factories do not run in unit test files (but in feature tests, after update 5.4>6)

My factories do not get found in unit test files, but only in feature tests.

Here is the exact same test with the respective class infos. I recently upgraded the app form Laravel 5.4 to 6.0. Maybe I need to adjust some test base classes or the like.

Factory

<?php

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

use Faker\Generator as Faker;

$factory->define(App\Note::class, function (Faker $faker) {
    return [
        'contact_id' => 1,
        'type' => 1,
        'body' => 'Long sentence',
    ];
});

Feature test file (passing)

<?php

namespace Tests\Feature\Http\Controllers;

use App\Note;
use Tests\TestCase;
use Illuminate\Foundation\Testing\RefreshDatabase;

class NoteControllerTest extends TestCase
{
  use RefreshDatabase;

/** @test */
    public function it_does_get_icon_name_for_note_type()
    {
        $note = factory(Note::class)->create();
        $this->assertEquals('Interaction', $note->type_description);
    }
}

Unit test file (failing)

<?php

namespace Tests\Unit;

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

class NoteUnitTest extends TestCase
{
    use RefreshDatabase;

    /** @test */
    public function it_does_get_icon_name_for_note_type()
    {
        $note = factory(Note::class)->create();
        $this->assertEquals('Interaction', $note->type_description);
    }
}

Error message: Unable to locate factory with name [default] [App\Note].

Of course I could keep all the unit tests in my feature test sets, but that would be the worst solution.