[REF] event(_*): clean tests common files

Purpose is to have a common event class for users and useful stuff (customers,
products, ...) but lessen usage of common test data through sub modules.
Indeed having a "global event type" test data updated in various addons is
actually complicated to maintain.

Sub add-ons are updated to use mainly the ``EventCase`` test class holding
users and side data. Data specific to those modules (event type with some
specific configuration notably) is created and used in tests in the given
module only, and not through generic event_type_complex and event_0 test
data anymore.

With this commit tests are more localized to their add-on and modifying data
in a given add-on has less chances to have unwanted side effect in other event
submodules unit tests.

Task-2703285 (Event performance improvements)
Task-2703289 (Event testing and coverage)

Part-of: odoo/odoo#81068