Solved it this way:

protected function mapApiRoutes()
    {
        $callback = function ($router) {
            require base_path('routes/api.php');
        };

        //production
        Route::group([
            'middleware' => 'api',
            'namespace' => $this->namespace,
            'domain' => 'api.{domain}.{tld}'
        ], $callback);

        //test
        Route::group([
            'middleware' => 'api',
            'namespace' => $this->namespace,
            'domain' => 'apitest.{domain}.{tld}'
        ], $callback);        
    }

source: https://github.com/laravel/framework/issues/4017#issuecomment-...

UPDATE. This is NOT solved. Well, not if I want to use parameters from the URI. For instance,

Route::get(test/{id}, function($id) {
    return $id     //example (where domain is api.example.local)
});

If I keep the code as is, I would have to use the route like this,

Route::get(test/{id}, function($domain, $tld, $id) {
    return $id   //999 (where id = 999)
});