ipatests: add suite for testing custom plugins
    
    Ensure that attributes and objectclasses are case-insensitive.
    
    https://pagure.io/freeipa/issue/8415
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>