Pytest warnings in test_icc.py

This bug was originally reported by at Jelle van der Waa (jelle@vdwaa.nl) in Debian bug #916226:

Running the tests with 4.0.1 produces the following error:

tests/comparators/test_icc.py:53: in cd_iccdump_version
    val = subprocess.check_output(('cd-iccdump', icc1().path)).decode('utf-8')
E   _pytest.warning_types.RemovedInPytest4Warning: Fixture "<lambda>" called directly. Fixtures are not meant to be called directly, are created automatically when test functions request them as parameters. See https://docs.pytest.org/en/latest/fixture.html for more information.
=============================== warnings summary ===============================