Update the AdAccountGroup call to correctly return adaccounts

Update the unit tests to correctly call gettting of adaccounts

The unit tests were failing for getAccounts, it was returning an unsupported get Request error

To get all adaccounts for an ad group use:
https://graph.facebook.com/AdAccountGroupId/adaccounts

see here:
https://developers.facebook.com/docs/reference/ads-api/adaccountgroup
this sounds wrong, is his not ad_accounts or something else more revertable?