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:	pytest-dev/pytest#11043
MFC after:	3 weeks
Sponsored by:	Orange Business Services
Differential Revision:	https://reviews.freebsd.org/D41064

libexec/atf/atf-pytest-wrapper/atf_pytest_wrapper.cpp

@@ -141,6 +141,8 @@ class Handler {
       std::vector<std::string> args = {"pytest", "-vv", "-p",
         "no:cacheprovider", "-s", "--atf"};
 
+      args.push_back("--confcutdir=" + python_path);
+
       if (flag_list) {
         args.push_back("--co");
         args.push_back(script_path);