Silence pytest.yield_fixture deprecation warning
    
    pytest >= 2.10 supports yield based fixtures with pytest.fixture. In
    pytest < 2.10 pytest.yield_fixture is required. But that function
    also raises a deprecation warning in pytest >= 3.0. FreeIPA has to stay
    compatible with pytest < 2.10 for RHEL 7 testing.
    
    https://docs.pytest.org/en/latest/fixture.html#fixture-finalization-executing-teardown-code
    
    https://pagure.io/freeipa/issue/6591
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>