[IMP] mail: tests: clean and improve mail channel tests

Purpose is to lessen noise and make tests less low-level. Including :

 * tests should be more about channel listeners than followers as
   channels should not have followers;
 * test email_send that trigger the mailing list behavior;
 * currently add some join and leave channel methods are those are
   missing in the base model:
 * test joining and leaving channels that should not change
   followers but members;
 * use less preconfigured data;
 * add some mute loggers to lessen noise;
 * remove some old code about patching that is not necessary anymore and
   could actually create funny bugs;

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