lib/libc/tests/string: add unit test for strlcpy

A straightforward derivation from the stpncpy unit test.

Sponsored by: The FreeBSD Foundation
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42863

(cherry picked from commit f7098b8659923873a7c60b64cb68182e470786f9)