consolidate pytest config in pyproject.toml

[jameslamb]

fixes #6194

Wheel tests in this project are emitting tons of warnings like this:

test_random_forest.py:1247
/__w/cuml/cuml/python/cuml/cuml/tests/test_random_forest.py:1247: PytestUnknownMarkWarning: Unknown pytest.mark.memleak - is this a typo? You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
@pytest.mark.memleak

I think that's because the introduction of a pytest.ini file in #6078 resulted in all of the pytest options from pyproject.toml being ignored.

From https://docs.pytest.org/en/stable/reference/customize.html#pytest-ini

pytest.ini files take precedence over other files, even when empty.

I think "take precedence" there means that if pytest finds a pytest.ini, it stops searching for other configuration files.

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[jameslamb]

/ok to test