webui tests: fix algo for finding available idrange
    
    The webui tests for ID range evaluate a potentially free id range
    by looking for existing ranges and picking a range = max value
    + 1 million.
    
    With the addition of subuid range this algorithm produces values
    over the limit because the subuid range goes from
    2,147,483,648 to 4,294,836,224 and the max base id is 4,294,967,295.
    
    Ignore the subuid range when picking a potential range.
    Fixes: https://pagure.io/freeipa/issue/8919
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>