lib/libc/tests/string: add test for strnlen()

Tests for strnlen, checks alignments from up to 16 and buffer sizes up
to 64, also checks that passing SIZE_MAX as maxlen works, because it can
cause a wraparound error if strnlen is incorrect.

Reviewed by: fuz, emaste (GSoC mentors), kib
Sponsored by: Google LLC (GSoC 2024)
Differential Revision: https://reviews.freebsd.org/D46275