lib/libc/tests/string/strcspn_test.c: add test for correct match order

This new unit test verifies that if there are multiple
matches, the first match is returned, ignoring later
matches.

Approved by: mjg (blanket, via IRC)
MFC after: 1 week
MFC to: stable/14

(cherry picked from commit a0ecf2224ea35d029d33541878f0eee42f5fd84f)