[3.13] gh-125346: Add more base64 tests (GH-141061) (GH-141130) (GH-141133)

(cherry picked from commit fd17ee026fa9b67f6288cbafe374a3e479fe03a5)

Add more tests for the altchars argument of b64decode() and for the map01
argument of b32decode().
(cherry picked from commit 13360efd385d1a7d0659beba03787ea3d063ef9b)

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