ui_tests: select_combobox() fixes
    
    Move strict "search_btn" element finding to later so we
    do not fail when using combobox without search button.
    Also switch open_btn.click() before fill_textbox() as it
    is used to close the selection.
    
    https://pagure.io/freeipa/issue/7441
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>