[FIX] website: prevent race condition on visitor merge test

Since [1], a race condition would be faced as the dict order is not
guaranteed, sometimes failing because:
```
AssertionError: Lists differ: ['/demo', '/admin'] != ['/admin', '/demo']
```

[1]: https://github.com/odoo/odoo/commit/a87b4142dd4a2c05e3e1885b2c54f5e0d3c7ac47

runbot-15727

closes odoo/odoo#111417

X-original-commit: 3e9a1dd76ce5172c4797bc92f0351e107589c3a5
Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>

addons/website/tests/test_website_visitor.py

@@ -437,7 +437,7 @@ class WebsiteVisitorTests(MockVisitor, HttpCaseWithUserDemo):
         self.assertFalse(Visitor.search_count([('partner_id', '=', self.partner_demo.id)]),
                          "The demo visitor should've been merged (and deleted) with the admin one.")
         # Track check
-        self.assertEqual(visitor_admin.website_track_ids.mapped('url'), ['/admin', '/demo'])
+        self.assertEqual(visitor_admin.website_track_ids.sorted('url').mapped('url'), ['/admin', '/demo'])
 
     def test_merge_partner_with_visitor_single(self):
         """ The partner merge feature of Odoo is auto discovering relations to