[IMP] test_mail: add test for auto subscribe

Manual backport of commit done in 13.0 odoo/odoo@ad210729da4bf72dad77acc4030a38478340f4c6

We recently found a bug about generic internal subtypes not being correctly
propagated when doing auto subscribe.

For example: subscribe to a project, check an internal subtype like activities.
Auto subscribe done on project tasks should include actitivities subtype but
is currently not the case.

This test highlights the issue and a fix will be provided soon. In this test
we check both

  * generic subtype propagation (comment- or activities- like subtypes);
  * model-specific subtype propagation through parent relationship (like
    following New Task on project to follow New on task);
  * internal subtypes not being propagated to portal users;
  * auto subscribe at create with both create and parent follower reasons;

LINKS

Task ID-2205643
Task ID-2241688
COM PR #56775

Co-Authored-By: Thibault Delavallee <tde@odoo.com>
Co-Authored-By: Priyanka kakadiya <pka@odoo.com>