test-path-utils: use xsnprintf in favor of strcpy

This strcpy will never overflow because it's copying from
baked-in test data. But we would prefer to avoid strcpy
entirely, as it makes it harder to audit for real security
bugs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>