[FIX] tests: ensure that previous user is logged off in authenticate

In some case, a browser_js test with login "None" following a browser_js
test with a defined login could result in the second test being executed
with the previous user.

This was caused by a race condition, a request response comming back
to chrome just after browser clear, restoring the old cookie.
(All odoo request have the set_cookie flag set in order to refresh
cookie timeout)

The solution here is to check one more time for cookie in authenticate,
but also to remove HTTPCase session from session_store. This will
only be effective when calling browser_js without login in the same
HTTPCase .

closes odoo/odoo#39424

Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>