lib/libc/tests/string: add unit tests for strcspn(3)
We currently use the NetBSD test suite to cover strcspn(3). It only
contains a very rudimentary test of this function. This all new set
of unit tests for the FreeBSD test suite should cover many more edge
cases relating to alignment issues.
Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41557
(cherry picked from commit 35a5359406fe779186d1fd7131c95927fefe20be)