[IMP] test_mail: add unit and performance tests about adding followers

Purpose of this commit is to test existing behavior of followers subscription
as well as adding some query count tests about message_subscribe method. As
followers subscription technical implementation may change in a near future
in order to be optimized it is a good idea to have current behavior and query
count validated.

This commit adds

 * test for followers update using message_subscribe API. Tests are added
   with forced and unforced subtypes to ensure behavior will not change or
   will be changed while knowing it;
 * performance test about message_subscribe, about adding new followers,
   and subscribing already-existing followers in force mode or not;

Query count performance test shows that message_subscribe could probably
be improved. It does several things that require queries (check of access
rights, check for existing subscriptions, ensure customers have no access
to internal subtypes, ...). However we feel it could be improved in order
to speed-up the subscription of followers.

This is linked to task 49265; it is only the first steps of it.

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