fixtures: fix --fixtures-per-test crash on fixture-using doctest

[bluetech]

Fix #15051

[kilisamemarisaaa]

I independently reproduced the reported crash on Windows and re-ran the exact MRE against this head.

Environment: Windows (win32), Python 3.12.6, pytest main 6a0de9be baseline. An autouse fixture plus test_doctest.txt makes --fixtures-per-test raise AttributeError: DoctestItem has no attribute function and exit 0. On head f648fa2e, the same command reports (test_doctest.txt:1), lists auto_fixture, and exits 0; the focused upstream regression file passes (10 passed). The ordinary doctest passes on both baseline and head.

The reportinfo()-based location is appropriate for both function and doctest items, and this does not change the successful command's exit status. Approving based on this Windows verification.