test_webui: test_hostgroup: Wait for modal dialog to appear
    
    Modal dialog transition is currently set to 300ms, we have to wait
    for it to appear in order to interact with it. Double that time is a
    safe value.
    
    Resolves: https://pagure.io/freeipa/issue/8684
    
    Signed-off-by: Michal Polovka <mpolovka@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>
    Reviewed-By: Serhii Tsymbaliuk <stsymbal@redhat.com>