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
actually this isn't quite right, in this case it should use the class AdAccount::className() which provides the correct endpoint to use.
I'll correct as I have some other changes to sync.