[FIX] mail, test_mail: fix mail_tour

This commit fixes the following issues regarding the mail tour:

1. Start from discuss app (not app switcher)
2. Adapt from recent changes on discuss app https://github.com/odoo/odoo/commit/9f6f9c71d3258ce3652b4a5793b7dcfef6f01d25

Note there is an error if we let runbot run this tour automatically:

```
Tour mail_tour failed at step Messages can be <b>starred</b> to remind you to check back later.
(trigger: .o_mail_discuss .o_mail_thread .o_thread_message_star)
```

This comes from the mail tour requiring longpolling to work, which is
intentionally disabled when running tours automatically on runbot.
We plan making this tour run without longpolling in the future, which
requires a design change from the client code (e.g. reduce reliance
on the longpolling).

Task-ID 1916093

closes odoo/odoo#29575

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