[FIX] calendar: test was trying to send mails for past events

Before this commit, the test was trying to send mail for a past event.
It would fail with the following error:

```
FAIL: TestCalendar.test_event_creation_mail
Traceback (most recent call last):
  File "/data/build/odoo/addons/calendar/tests/test_calendar.py", line 329, in test_event_creation_mail
    _test_one_mail_per_attendee(self, partners)
  File "/data/build/odoo/addons/calendar/tests/test_calendar.py", line 311, in _test_one_mail_per_attendee
    self.assertEqual(len(mail), 1)
AssertionError: 0 != 1
```

https://runbot.odoo.com/web/#id=29871&view_type=form&model=runbot.build.error&menu_id=405&cids=1

closes odoo/odoo#141411

X-original-commit: 2b0bedf5a57ca2733501359fb8cc8e5bc743a4fa
Signed-off-by: Arnaud Joset (arj) <arj@odoo.com>

addons/calendar/tests/test_calendar.py

@@ -7,6 +7,8 @@ from datetime import date, datetime, timedelta
 from odoo import fields, Command
 from odoo.tests import Form, HttpCase, tagged
 from odoo.addons.base.tests.common import SavepointCaseWithUserDemo
+
+import freezegun
 import pytz
 import re
 
@@ -296,8 +298,10 @@ class TestCalendar(SavepointCaseWithUserDemo):
 
         self.assertEqual(str(activity_id.date_deadline), '2018-10-16')
 
+    @freezegun.freeze_time('2023-10-06 10:00:00')
     def test_event_creation_mail(self):
         """
+        Freezegun used because we don't send mail for past events
         Check that mail are sent to the attendees on event creation
         Check that mail are sent to the added attendees on event edit
         Check that mail are NOT sent to the attendees when the event date is past