TESTS: Adding tests for ad_enabled_domains option
    
    There is special logic around ad_enabled_domains option:
     * option is disabled by default
     * master domain is always added to enabled domains
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2828
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>