Prior to this commit, starting this test individually would not work as
the admin user is subscribed to the mailing list used during the test.

The reason the test has passed our test suite is because on runbot the
`mass_mailing_sms` module is installed, which adds a new mailing list to
 which the admin user is not subscribed.

 This commit changes the test to always ensure that the admin is not
 subscribed to any newsletter, making the test a bit more robust.

closes odoo/odoo#118869

Signed-off-by: Outagant Mehdi (mou) <mou@odoo.com>