pytest: Migrate unittest/nose to Pytest fixtures
    
    Even though Pytest supports xunit style setups, unittest and nose
    tests, this support is limited and may be dropped in the future
    releases. Worst of all is that the mixing of various test
    frameworks results in weird conflicts and of course, is not widely
    tested.
    
    This is a part of work to remove the mixing of test idioms in the
    IPA's test suite:
    1) replace unittest.TestCase subclasses
    2) replace unittest test controls (SkipTest, fail, etc.)
    3) replace unittest assertions
    
    Related: https://pagure.io/freeipa/issue/7989
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>