libc/nss tests: Fix getpw and getgr single-pass tests

Some NSS regression tests for getgrent(3) and getpwent(3) were not
testing anything because the test incorrectly requested creation of a
database snapshot.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 7abc10098b3d9c23d611294714b3058e8048aec0)