Unit testing with the config app file in Laravel

My model method relies on the config() global, here;

public function getGroup()
{
    if(config('app.pages.'.$this->group.'.0')) {
        return $this->group;
    }
    return "city";
}

I am trying to test this method in my unit test class, here;

public function testGetGroupReturnsCityAsDefault()
{
    $response = new Response();
    $response->group = "town";
    $test = $response->getGroup();
    dd($test);
}

The error I get is;

Error: Call to a member function make() on null
/home/vagrant/sites/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php:62
/home/vagrant/sites/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php:163

I know this is related to the config() global. But not sure how to set it in my test. I tried

public function setUp()
{
config(['app.pages' => [
    'city' => [........

But got the same error. How can I set this up?