t-reftable-stack: use Git's tempfile API instead of mkstemp()

t-reftable-stack: use Git's tempfile API instead of mkstemp()

Git's tempfile API defined by $GIT_DIR/tempfile.{c,h} provides
a unified interface for tempfile operations. Since reftable/stack.c
uses this API for all its tempfile needs instead of raw functions
like mkstemp(), make the ported stack test strictly use Git's
tempfile API as well.

A bigger benefit is the fact that we know to clean up the tempfile
in case the test fails because it gets registered and pruned via a
signal handler.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>