gh-94808: Improve coverage of _PyBytes_FormatEx (GH-95895)

There were two specific areas not covered:

- %(name) syntax
- %*s syntax

Automerge-Triggered-By: GH:iritkatriel
(cherry picked from commit dde15f5879c3576db42ee4366fb684747c31459f)

Co-authored-by: Michael Droettboom <mdboom@gmail.com>