lib/libc/tests/string: add unit tests for strncmp(3)

These are patterned after the previously added (D41970)
strcmp tests, but are extended to check for various length
conditions.

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/D42122