Issue 51005 - AttributeUniqueness plugin's DN parameter should not have a default value
    
    Bug Description:
    Currently we have an optional dn parameter with a default value:
    
    def __init__(self, instance, dn="cn=plugins,cn=config"):
    
    This breaks some tests that assume the default base DN for this plugin.
    But it can have multiple instances, so we should always specify the DN.
    Making this parameter mandatory will ensure this.
    
    Fix Description:
    Remove the default DN value and update tests.
    
    Fixes: https://pagure.io/389-ds-base/issue/51005
    
    Reviewed by: mreynolds (Thanks!)