[FIX] mail: wrong parameter type for add_members

The method 'add_member' has 'partner_ids' as a optional parameter , but
the thing is the test 'test_attachment_hijack' is only passing it as a
single ID off res.partner record. Let's say if somewhere we overide this
method and write something like
self.env['res.users'].search([('partner_id', 'in', partner_ids)]) , it
will fail because 'partner_ids' should be a list.

closes odoo/odoo#128759

X-original-commit: 75cb1266afa8b921f8d0ca153f5d05ad47c9b14c
Signed-off-by: Vranckx Florian (flvr) <flvr@odoo.com>

addons/mail/tests/discuss/test_message_controller.py

@@ -178,7 +178,7 @@ class TestMessageController(HttpCase):
         demo = self.authenticate("demo", "demo")
         channel = self.env["discuss.channel"].create({"group_public_id": None, "name": "public_channel"})
         channel.add_members(
-            self.env["res.users"].browse(demo.uid).partner_id.id
+            self.env["res.users"].browse(demo.uid).partner_id.ids
         )  # don't care, we just need a channel where demo is follower
         no_access_request = self.url_open("/web/content/" + str(att.id))
         self.assertFalse(