Improve link tracker tests :
* add missing test for trace tracking in mass mailing. We generate a trace
and find a short link in it, in order to test the tracking in mass
mailing;
* add tests for side effects of mass mailing routes: update of trace status,
clicks, ...
* wrap base url locally instead of globally in MockTracker class to ease
specific setup in classes;
Prepares Task-2150462 (Mass Mailing: Unsubscribe flow improvement)
closesodoo/odoo#94660
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>