[FW][FIX] event : Changing seats_max could desync seates_reserved

Issue: Sometimes, when changing the Maximum (seats_max) on one of
the tickets of an event, it triggered a recompute for the other tickets

Steps to reproduce :

Install Events
Settings > Event > Enable "Tickets"
Create an event template (or use Sell Online default one) with
Check "Ticketing" and set the line price to 0
Create an event :
with that template
and Autoconfirm checked
Add a line for the Tickets:
name: VIP
price: 10
Save the event

Create two attendees for the event, one for each
Event Ticket (event_ticket_id) and confirm them (on the form, not
Confirm Attendee)

Change the Maximum (seats_max) of one ticket and save
-> the Confirmed (seats_reserved) will be recomputed but the
confirmed for the other ticket will increase as well

Side-Note:
I haven't been able to find a deterministic way to reproduce the bug
but it seems that the bug appear the most when doing all the steps at
once, and trying to not log out or refresh the page.
Also it works best on a runbot or at least with runbot data.

Without my modification, the new test passes on my local odoo server,
but fails on a dump of a runbot on my computer, adding my modification
makes it work on either case

Why is that a bug:
The recomputation seems to fail for some reason, we are setting all
the event/ticket in self to 0, but only update the value of those by
fetching a SQL query so there might be a desync there

opw-2642555

closes odoo/odoo#77670

Forward-port-of: #76492
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>