Laravel | Testing

I'm writing a http test for laravel 5.6. After test I've got a warnig "Test code or tested code did not (only) close its own output buffers".

In a tested method I have a Mail send method. I've figured out, z the error comes from this method.

The code of send email is

public static function onRequestPasswordChangeSendResetLink(User $user, $resetUrl)
{
        Mail::to($user->email)
            ->send(new PasswordChange($user, $resetUrl));
}

What is wrong?)