[3.12] gh-108082: C API: Add tests for PyErr_WriteUnraisable() (GH-111455) (GH-111507)

Also document the behavior when called with NULL.
(cherry picked from commit bca330542912532baa33af20a107fcf956cf007a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>