test-hashmap: use xsnprintf rather than snprintf

test-hashmap: use xsnprintf rather than snprintf

In general, using a bare snprintf can truncate the resulting
buffer, leading to confusing results. In this case we know
that our buffer is sized large enough to accommodate our
loop, so there's no bug. However, we should use xsnprintf()
to document (and check) that assumption, and to model good
practice to people reading the code.

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