[3.12] gh-74678: Increase base64 test coverage (GH-21913) (GH-113811)

Ensure the character y is disallowed within an Ascii85 5-tuple.

(cherry picked from commit 802d4954f12541ba28dd7f18bf4a65054941a80d)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>
Co-authored-by: Lee Cannon <leecannon@leecannon.xyz>