Fix login access when sign_up is disabled

Add tests for checking whether links to "Log In" or "Sign Up" should be
shown, depending on the configuration of the registration.

Add missing cassette for denied registrations.

Co-authored-by: Eduardo Navarro <enavarro@suse.com>
Co-authored-by: Saray Cabrera Padrón <scabrerapadron@suse.de>