Testing of presence of SameSite and Secure cookies doesn't work

[Adrien Carpentier]

When using the following constants in settings.py, as Django doc says (​https://docs.djangoproject.com/en/3.1/ref/settings/#std:setting-SESSION_COOKIE_SECURE):

CSRF_COOKIE_SECURE = True
SESSION_COOKIE_SECURE = True
CSRF_COOKIE_SAMESITE = 'None'
SESSION_COOKIE_SAMESITE = 'None'

...and then when testing the presence of SameSite and Secure cookies in the responses, there is no SameSite neither Secure cookie keys.
Here is a non passing test, for example, for a user agent that should have SameSite and Secure cookies:

agent_string = "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.2227.0 Safari/537.36"
from django.test import Client
test_client = Client()
res = test_client.get("/", HTTP_USER_AGENT=agent_string)
assert res.cookies.get(self.cookie_key)["samesite"] == "None"
assert res.cookies.get(self.cookie_key)["secure"]

When printing the content of the cookies (print(res.cookies.items())), the cookie keys are not there.

(Until 3.1 I was adding SameSite and Secure cookies in the responses through a custom middleware before Django 3.1, depending on the user agent, with the exact same passing tests. Since Django 3.1, I just removed the custom middleware and added those constants in settings.py).