Add test for new Contact,get group result format

It seems that in 4.7 the result of

civicrm_api3('Contact', 'get', ['id' => $id, 'return' => group]);

has changed. It's an improvement IMO.

I think this may have sneaked in unnoticed. I plan to add a test to lock down the new behaviour, since the 4.6 format was a bit ungainly (you can't tell if you're looking at two group titles, or a group title with a comma in it).

Array
(
    [is_error] => 0
    [version] => 3
    [count] => 1
    [id] => 3
    [values] => Array
        (
            [3] => Array
                (
                    [contact_id] => 3
                    [groups] => 1,2
                    [id] => 3
                )

        )
)