lib/libc/tests/string/strcspn_test.c: extend tests to catch previous bug

This extends the strcspn() unit tests to catch mistakes in the
implementation that only appear when a mismatch occurs in a certain
position of the string against a certain position of the set.

See also: 52d4a4d4e0dedc72bc33082a3f84c2d0fd6f2cbb
Sponsored by: The FreeBSD Foundation
Approved by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41821