How to test authentication via API with Laravel Passport?

I'm trying to test the authentication with Laravel's Passport and there's no way... always received a 401 of that client is invalid, I'll leave you what I've tried:

My phpunit configuration is the one that comes from base with laravel

tests/TestCase.php

abstract class TestCase extends BaseTestCase
{
    use CreatesApplication, DatabaseTransactions;

    protected $client, $user, $token;

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

        $clientRepository = new ClientRepository();
        $this->client = $clientRepository->createPersonalAccessClient(
            null, 'Test Personal Access Client', '/'
        );
        DB::table('oauth_personal_access_clients')->insert([
            'client_id' => $this->client->id,
            'created_at' => date('Y-m-d'),
            'updated_at' => date('Y-m-d'),
        ]);
        $this->user = User::create([
            'id' => 1,
            'name' => 'test',
            'lastname' => 'er',
            'email' => 'test@test.test',
            'password' => bcrypt('secret')
        ]);
        $this->token = $this->user->createToken('TestToken', [])->accessToken;
    }
}

tests/Feature/AuthTest.php

class AuthTest extends TestCase
{
    use DatabaseMigrations;

    public function testShouldSignIn()
    {
        // Arrange
        $body = [
            'client_id' => (string) $this->client->id,
            'client_secret' => $this->client->secret,
            'email' => 'test@test.test',
            'password' => 'secret',
        ];
        // Act
        $this->json('POST', '/api/signin', $body, ['Accept' => 'application/json'])
        // Assert
        ->assertStatus(200)
        ->assertJsonStructure([
            'data' => [
                'jwt' => [
                    'access_token',
                    'expires_in',
                    'token_type',
                ]
            ],
            'errors'
        ]);
    }
}

My handy authentication with passport for testing purposes

routes/api.php

Route::post('/signin', function () {
    $args = request()->only(['email', 'password', 'client_id', 'client_secret']);
    request()->request->add([
        'grant_type' => 'password',
        'client_id' => $args['client_id'] ?? env('PASSPORT_CLIENT_ID', ''),
        'client_secret' => $args['client_secret'] ?? env('PASSPORT_CLIENT_SECRET', ''),
        'username' => $args['email'],
        'password' => $args['password'],
        'scope' => '*',
    ]);
    $res = Route::dispatch(Request::create('oauth/token', 'POST'));
    $data = json_decode($res->getContent());
    $isOk = $res->getStatusCode() === 200;
    return response()->json([
        'data' => $isOk ? [ 'jwt' => $data ] : null,
        'errors' => $isOk ? null : [ $data ]
    ], 200);
});

via Chebli Mohamed