[3.13] gh-121084: Call _abc_registry_clear() when checking refleaks (GH-121191) (#121208)

gh-121084: Call _abc_registry_clear() when checking refleaks (GH-121191)

dash_R_cleanup() now calls _abc_registry_clear() before calling again
register().
(cherry picked from commit c766ad206ea60b1e0edcb625b99e7631954a984f)

Co-authored-by: Victor Stinner <vstinner@python.org>