[IMP] test_website: add a test for 308 redirect with multi url route

This commit introduces a test in Odoo 15 about a broken behavior in Odoo
16 which was not covered by a test.
A fix will be shipped alongside this commit forward-port in Odoo 16.

The behavior involved here is about:
- Having a multi-url defined route
- One of those URL not having a model converter
- One of those URL having a model converter
- Creating a 308 redirect for the URL without model converter

Something like this route (real example from the code):
```py
@http.route([
    '''/shop''',
    '''/shop/page/<int:page>''',
    '''/shop/category/<model("product.public.category"):category>''',
    '''/shop/category/<model("product.public.category"):category>/page/<int:page>'''
], type='http', auth="public", website=True, sitemap=sitemap_shop)
```
And this redirect (real usual example from clients):
```py
self.env['website.rewrite'].create({
    'name': 'Test Multi URL 308',
    'redirect_type': '308',
    'url_from': '/shop',
    'url_to': '/magasin',
})
```

In Odoo 15, everything will work as expected:
1. Accessing /shop will redirect to /magasin
2. Accessing /shop/category/categ-1 will not redirect and user will be
   on /shop/category/categ-1 still

But in Odoo 16:
In the case described above, acessing the URL with model converter from
the route will fail and redirect to a non slugified URL in the form of
`/some-url?param=test.model%287%2C%29` which is basically the
stringified encoded version of the record `test.model(7,)`.

Note:
- It doesn't matter if the route is contained in the other, the error
  will still occur even if `'''/shop'''` was `'''/abc'''`.
- Even if all URL from the route have their own 308 redirect, the error
  will still occur.
- It won't have any issue if there is a 308 on the route with model
  converter: both the one without model converter and other url with
  model converter will work and won't be impacted.

This might be because of a concept of "merging URL" from httpocalypse,
see https://github.com/odoo/odoo/commit/347a3ccf763cc6958d5dc1df3168c9c65245736c

opw-3450054
opw-3466498
opw-3477380
opw-3475694

X-original-commit: 9230f7db60b10ca2a3e83d6beefb6cf72d09a804
Part-of: odoo/odoo#133371