pytest: Warn about unittest/nose/xunit tests
    
    This Pytest plugin is intended to issue warnings on collecting
    tests, which employ unittest/nose frameworks or xunit style.
    For example, this may look like:
    """
    test_a/test_xunit.py:25
      test_a/test_xunit.py:25: PytestDeprecationWarning: xunit style is deprecated
    
        def test_foo_bar(self):
    
    test_b/test_unittest.py:7
      test_b/test_unittest.py:7: PytestDeprecationWarning: unittest is deprecated
        def test_foo_bar(self):
    """
    
    To treat these warnings as errors it's enough to run Pytest with:
    -W error:'xunit style is deprecated':pytest.PytestDeprecationWarning
    
    Related: https://pagure.io/freeipa/issue/7989
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>