t-reftable-stack: add test for non-default compaction factor

t-reftable-stack: add test for non-default compaction factor

In a recent codebase update (commit ae8e378430, merge branch
'ps/reftable-write-options', 2024/05/13) the geometric factor used
in auto-compaction of reftable tables was made configurable. Add
a test to verify the functionality introduced by this update.

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>