libc/tests: Clean up *dir() tests

Mainly, avoid reusing the name of one of the functions we should be
testing (but aren't) for local variables.

Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55054

(cherry picked from commit d70b9eb74fc4aa430bd2ff4bca37e6a9b6c8004f)