Some Orange tests fail when the add-on is installed

[markotoplak]

Orange.widgets.evaluate.tests.test_owtestandscore and Orange.widgets.evaluate.tests.test_owpredictions fail because they also use Fairness metrics. Metrics should me made more robust so that they cause no failures. Examples:

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ERROR: test_multi_target_input (Orange.widgets.evaluate.tests.test_owtestandscore.TestOWTestAndScore)
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Traceback (most recent call last):
  File "/home/marko/dev/orange3/Orange/widgets/evaluate/tests/test_owtestandscore.py", line 764, in test_multi_target_input
    self.send_signal(widget.Inputs.train_data, data)
  File "/home/marko/dev/orange-widget-base/orangewidget/tests/base.py", line 445, in send_signal
    return self.send_signals([(input, value)], *args,
  File "/home/marko/dev/orange-widget-base/orangewidget/tests/base.py", line 471, in send_signals
    self._send_signal(widget, input, value, *args)
  File "/home/marko/dev/orange-widget-base/orangewidget/tests/base.py", line 490, in _send_signal
    notify_input_helper(input, widget, value, *args, **kwargs)
  File "/usr/lib/python3.10/functools.py", line 889, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/home/marko/dev/orange-widget-base/orangewidget/utils/signals.py", line 735, in set_input_helper
    handler(*args)
  File "/home/marko/dev/orange-widget-base/orangewidget/utils/signals.py", line 208, in summarize_wrapper
    method(widget, value)
  File "/home/marko/dev/orange3/Orange/widgets/evaluate/owtestandscore.py", line 451, in set_train_data
    self._update_scorers()
  File "/home/marko/dev/orange3/Orange/widgets/evaluate/owtestandscore.py", line 518, in _update_scorers
    new_scorers = usable_scorers(self.data.domain)
  File "/home/marko/dev/orange3/Orange/widgets/evaluate/utils.py", line 84, in usable_scorers
    candidates = [
  File "/home/marko/dev/orange3/Orange/widgets/evaluate/utils.py", line 87, in <listcomp>
    and scorer.is_compatible(domain_or_var) and scorer.class_types]
  File "/home/marko/venv310/lib/python3.10/site-packages/orangecontrib/fairness/evaluation/scoring.py", line 33, in is_compatible
    return contains_fairness_attributes(domain)
  File "/home/marko/venv310/lib/python3.10/site-packages/orangecontrib/fairness/widgets/utils.py", line 105, in contains_fairness_attributes
    if "favorable_class_value" not in domain.class_var.attributes:
AttributeError: 'NoneType' object has no attribute 'attributes'