test_ipaserver.test_ldap: Adjust tests to Python 3's KeyView
    
    In Python 3, the keys() method of mappings returns a KeyView object
    that reflects the mapping's state. In LDAPEntry, this means that
    the collection returned by keys() is case-insensitive and supports
    aliases.
    
    Part of the fix for: https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>