lib/libc/tests/string: derive strspn(3) tests from strcspn(3) tests

To cover the new optimised amd64 strspn(3) SIMD implementation, extend
the previously written strcspn(3) unit test to also cover strspn(3).

Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41567