[MOV] mail, test_mail: move activity menu test to test_mail

In preparation for the adaptation of its adaptation to the new
way to seed tests data, the `systray_activity_menu_test` is moved to test_mail. Indeed,
with this adaptation, all fake models will be replaced by real ones, the `MailTestActivity`
model is the perfect candidate for this.

task-2792108

Part-of: odoo/odoo#87743

addons/test_mail/static/tests/helpers/model_definitions_setup.js

@@ -2,4 +2,4 @@
 
 import { addModelNamesToFetch } from '@mail/../tests/helpers/model_definitions_helpers';
 
-addModelNamesToFetch(['mail.test.track.all']);
+addModelNamesToFetch(['mail.test.track.all', 'mail.test.activity']);