atf_pytest_wrapper: fix use with pytest-7.4

As of pytest 7.4 it no longer walks all the way to the root directory
of the file system to find conftest files. As a result we don't find /
usr/tests/conftest.py, and don't load atf_python. That in turn causes
atf_python tests to fail.

Explicitly set the confcutdir, as advised by the pytest changelog.

See also: https://github.com/pytest-dev/pytest/pull/11043
MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D41064