[3.13] Refactor test_capi.test_long (GH-122113) (GH-122137)

Share common code for tests for PyLong_As*() functions.
(cherry picked from commit 69f2dc5c06e62b4a9eb4da8f0cd456cc09b998ed)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>