ec9c27d ui_tests: add assert_field_required()

Authored and Committed by mreznik 6 years ago
    ui_tests: add assert_field_required()
    
    Add assert_field_required() to check whether we
    got 'Required field' error message.
    
    https://pagure.io/freeipa/issue/7441
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>