mailing group tests were only passing because they were directly including the file

I have fixed the API being tested such that they reflect the BAO structure & work via the wrapper both as they did (or didn't) previously AND according to API standard call format - I updated the tests to use the API standard call format too.

This doesn't confirm whether they are working when called from core as

1) core is using a different (again) call pattern
ie.
civicrm_api('MailingGroup', 'event_unsubscribe', $params);

vs TESTED (& failing)
civicrm_api('mailing_group_event', 'subscribe', $params);

vs now tested & working
civicrm_api('mailing_event_subscribe', 'create',$params);

(I've always thought the api would only support single word actions - but maybe someone has changed that)

2) The tests are mostly failure tests anyway -

3) I only touched the functions that had tests (as I am trying to keep the tests working )

SO, we need to figure out a way to establish if the core is working properly - hopefully there are tests somewhere else that will check this for us?