Make them easier to improve and modify
* use a dedicated setup (allowing to add specific unit tests on test data);
* move initial asserts into its own unit test (to keep other tests shorter);
* use available mocks for freezetime and sql.now;
Then add tests for registration emails, to check what happens for communication
scheduled right at registration time.
Task-3084943: Event: Improve communication scheduler scalability
Part-of: odoo/odoo#155717
Co-authored-by: Stéphane Debauche <std@odoo.com>
