test: add tests for descriptive error message in ipa user-add
    
    Add tests for renaming existing user and group with invalid name or only numeric name,
    add numeric-only stage user, rename some functions and fix indentation
    Related: https://pagure.io/freeipa/issue/9378
    
    Signed-off-by: Erik Belko <ebelko@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>